This is a comparison of small vs large riders, comparing a  5 and 6 foot tall rider.  I originally wrote this while taking post bac classes at Texas A&M.

First of all lets assume similar body composition

between a 5 foot tall cyclist, and a 6 foot tall rider( or 1.2 times as tall as the other rider). 

 Strength is largely dependent on a rider’s area.  Area increases with the square of dimensions (1.2*1.2=

1.44 ) so the 6 footer has 1.44 times as much area when

Compared to the 5 footer.   Physiologists use cross

Sectional   area of a muscle to determine strength.

So the 6 footer is expected to be %44 stronger.

Volume increases  with the cube of the linear

dimensions.(1.2*1.2*1.2=1.728) so the  6 footer has

about %73 more volume. If they are of similar body

composition, the 6 footer is %73 heavier.

So the 6 footer is %73 heavier, but only %44 stronger.

Does this put the heavier rider at a disadvantage?

Yes and no.

We all know that when pedaling at speed in the flats,

air/wind is the primary force the cyclist must

overcome.   We also know rather well that as the road

turns uphill, and speeds slow down, the primary force

begins to change from air to gravity.

When pedaling uphill, the larger rider is obviously at

a disadvantage, as since his weight is %73 heavier,

but his engine is only %44 percent stronger.  In a

hilly 98 mile stage of the 1992 Tour Dupont, the

average 140 pound cyclists finished 10 minutes in

front of the average 175 pound cyclist

When pedaling in the flats with no drafting, the

larger rider is at an advantage, because  wind

resistance is in relation to surface area.    Since

surface area squares with linear dimensions, and the

volume cubes with linear dimensions, the heavier rider

has less surface area compared to their weight.  This

means that even though a larger rider requires more

energy to push through the wind, the energy costs

relative to weight are much less.

Research has indicated that larger cyclists require

%20 less oxygen per kilogram of body weight to ride in

the flats in a tucked aero position!  (This is

partially because of the fact that the big guys were

able to reduce their wind resistance in an aero tuck

by a greater percentage of total when compared to the

small guys)   This would predict that all things being equal, on a flat

time trial, the larger rider would win every time.  Of

course this doesn’t always happen as riders have a

large variance in vo2max, LT, muscle fiber type,

motivation, pain threshold, etc.    One study of elite

cyclists suggested that a 175 pound rider would finish

a flat 40k tt 4 minutes faster than a 145 pound rider.

Basically, smaller riders tend to have a more favorable strength to weight

 ratio, while larger riders have a more favorable strength to wind resistance

ratio.

Why are some riders like Greg Lemond or Lance

Armstrong at the top of their game in tt and climbing?

 They are pretty much just genetic anomalies with very high power outputs .

* Most of this info is plagiarized directly from Ed