I feel like every time I watch an inside look at a pro bike whether it be on GCN or any other cycling channel the pros are selecting smaller sizes. The one I watched this morning the rider was nearly 6 feet but riding at 54 where I think ya mere mortals would ride a 56. And I’m pretty sure Froome rides a 56 and he is a few inches over 6 feet. So the question is why? Are the suggested sizes wrong? Are they getting some advantage?
Lower stack height. Us mere mortals don’t have the flexibility to get that low. You’ll see pro’s on a small frame with ridiculously long stems to handle their arm length and offset seatposts
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^^^yeah it was told to me the head tubes being so tall on stock bikes they fit more due to the tall head tubes (for pros).
Aero.
I chatted with a pro about this a few years ago over a cinnamon roll, talked about some of the bonkers stuff some riders demand which makes no sense. 5-10*+ of saddle tilt, 140mm stems, 140psi in tires, and that basically all of it is ignorance and defiance of change. If you grow-up riding a tiny bike and miserable riding position, that will feel normal and when you have the authority of a pro, you don’t change. It was also interesting to hear about saddle sponsors complaining about riders spray-painting logos onto “their” saddles.
Also, I didn’t look at the numbers on the F10, but some bikes have peculiar dimensions and if you’re selecting a bike around maximum stack and reach numbers, you can only go down rather than up in frame size numbers.
I heard heard stiffness was a factor. A smaller frame will be stiffer than a larger frame because of the shorter tubes. There’s apparently only so much that can be done with layups at these larger companies to make the bigger bikes stiff enough and even then they’re still more flexy than their smaller counterparts. Remember the world tour (generally) has to ride stuff that we can buy off the shelf. They can then put on whatever custom stem and seatpost combination they please to get the position they’re looking for. I think I remember seeing a video on GCN where they were looking in a world tour mechanic’s tool chest and they had stems in 1mm increments instead of 10.
Speaking of custom builds for larger frames, I just took a tour of the Parlee shop in Beverly, MA and they were doing a custom build for a New England Patriots line backer in which they had to re-think even their stiffest layup to make a bike to support this 325 lbs 6 ft 7 in guy (if I remember correctly, but in short, huge). The head tube on that thing was about as long as my forearm. That might be a fringe case but it doesn’t seem to be the easiest to get long tubes properly stiff.
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Stiffer, lighter, nimbler. I’d imagine as carbon tech gets better, you’ll see more pros on bikes that are “better” suited to their best fit. But generally, if you can fit onto a smaller frame with stem, bar, saddle, seatpost, and cleat nuances, then go with a smaller frame.
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It’s all just front end height. The guys who need to get as low as they can (classics riders, domestiques) tend to ride the smallest frame they can fit on, to get the bars low enough so that they can draft off of some guy five inches shorter at 50kph.
The gc guys and climbers tend to ride higher front ends because the position that’s best for a Netherlands wind battle isn’t the best for riding up the cols. Even Froome – yes, he rides a small bike, but for goodness’ sake, if that’s an aero position, then wind tunnels help us all.
A few years ago, when my wife took her exchange group to Germany, I got the chance to stay with a guy who designs the bikes for a pro tour sponsor, and as a result works with the team closely (quite by accident – his wife was a partner teacher for mine). What bikes did Pro Tour riders choose? Some of the guys chose the “comfort” bike. Some guys chose the aero bike. Some guys chose bikes two sizes small to get their drops almost to the brake calipers. Some guys rode the same size bike as a Cat 1 or 2 the same age and build might.
As for stiffness, for pros and amateurs alike, just about any stock frame draped with 105 or higher is stiff enough for you unless you’re Andrei Griepel. That’s all in your head.
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A lot of it is down to fit and what they feel comfortable on.
IIRC Aru rides a 54 frame with a 100mm stem, basically out of the box bike. Sagan rides a custom sized Spesh with a 58 toptube for and a 52 size headtube. Cavendish rode a 48 Cervelo S5 to get as aero as possible but at same time Cervelo bikes tend to be shorter and taller for a given size.
Personally I dislike a short stem and like a fairly low front end, plus quite a lot of saddle setback. At 5’10" a 52 bike (Cannondale CAAD) with a couple of spacers and a 110mm stem works well for fit and handling. Could happily go another 10mm longer and take spacers out if was riding road more regularly.
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Very interesting, thank you all for the anwsers!!
I joined a Facebook group for Trek Domane owners. Having been shown stupid saddle to bar drop photos over so many years it was refreshing to see that the average rider wasn’t trying to perform extreme yoga every time they got on their bike. I’ve had a full 3D bike fit at a university sports science department and have barely a 2cm saddle to bar drop and a fairly high stack. But I’m comfortable and injury fee for 2 years. And aero as a barn door 
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It’s not even the pros who choose smaller frames. A lot of amateur and elite folks out there riding smaller frames. You can stretch the smaller frame out, not so much with a larger frame.
Any problems with feet catch front wheel badly though?
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That would be me. When I look at the manufacturers suggestions, I should be going for one size larger than I normally do.
If I do that however, the frame feels too big for me.
Taking Pinarello for instance, where I ought to be riding size 50, and I have in the past. However, that never felt comfortable and I have since then, switched to 46.5 instead.
Not, if you go shopping in one and need to do a LOT of 180 degree tight turns.
When that happens, you buy Mamachari. 
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RE: 5’10" and riding a 52 - yes my toes overlap the front wheel, but they also did that on a 54.
I’ve touched the wheel maybe once in 3 years on that bike though and that was correcting a track stand (was taught to always turn wheel direction of front foot).
Good pedal discipline (outside foot down or inside foot forward) means overlap shouldn’t be an issue. Where it can be an issue is fixed gear bikes with no freehub, but if you use that setup in the street that’s your lookout. Is not something I would do personally.
Please dont tell me that people that own domane try to slam that stem with higher seat post?
I bet you can breathe better with higher stack and happy hips with more open angle yeh? But its crap riding headwinds though 
Absolutely, that’s why I will not go lower than 56 or 54 depending on bike brand.