One possible theory is that there is an optimum design of frame and wheels, and as you increase the size of the person, you would expect the size of the frame to increase proportionally. Like any item of clothing. However, in reality when you look at frame sizes for any given model, you notice that they don't just scale up. The rear chainstay tends to stay fixed, but the top tube and headtube angle changes. Larger frames have steeper headtube angles. For example, the 26" version of the Surly Long Haul Trucker in size 42 has a Headtube angle of 70 degrees, whereas the 62cm frame has 71.5 degrees. On a Trek 520, the size 48cm has a headtube angle of 69.5 and size 63cm has 72.3 degrees.
One way to understand this might be that in each case, the wheels are fixed size, and for each size, the frame designer tries to make the frame as stiff and light as possible to accommodate the wheels.
|2020.06.05||Maximize stiffness for a given wheel size||Steve Grant|
|2020.06.05||Smaller wheels||Carol Evenchick|
|2020.06.05||Center of gravity - Body positioning||Steve Grant|