I can’t speak from personal experience yet, but I think I get the point behind this logic. I think it’s biomechanics with respect to the proportion of the rider to the unicycle, though please note that I have only a limited knowledge in this regard and lots of the points I will make are a mere guesswork. So take the following with a grain of salt. As an example, I’d use a couple of edge cases. Let’s consider two people:
- My wife who’s 5’1" (155 cm) tall.
- An acquaintance of mine (let’s call him Adam) who’s around 6’9" (205 cm) tall.
Now, the biomechanical system containing my wife seated on a 20" unicycle with 130 mm cranks will have certain features. The center of gravity of the system will be positioned not that far above the wheel. With properly adjusted seat height, during riding, the angle between her femurs and tibias will oscillate between a large obtuse angle (alpha) at 6 o’clock position and quite sharp acute angle (beta) at 12 o’clock position, with angle (gamma) at 9 o’clock position being quite sharp as well, resulting in wide range of available torque (relative to height and weight of the entire system), and, as a result, in a greater degree of control of the unicycle.
If you substitute Adam for my wife in the unicycle system, the features are going to change significantly. The center of gravity will be far above the wheel. With properly adjusted seat height, the alpha angle should be the same, but the beta and gamma angles are going to be much greater, resulting in quadriceps muscles being much less extended at the 9 o’clock position than in the case of my wife. This should, in my opinion, result in much smaller range of relative torque available to Adam at 9 o’clock position in comparison with my wife, resulting in relatively lesser degree of control over the unicycle.
In order for the rider-unicycle system to have the same features for my wife and Adam, you would have to seat Adam on a 26.5" uni with 170 mm cranks. In that case, the abovementioned factors would be the same, though Adam would have to deal with greater momentum of the wheel. This could, however, be offset with higher lateral stability associated with larger speeds associated with 26.5" in comparison with the 20". The only clear advantage I perceive for Adam in 20" wheel is the fact that it would be a lot safer for him than larger wheel, as the potential falls would be from a relatively small height.
So, based on this train of thought, I am positive that for taller riders, usage of larger wheels should produce similar results as smaller wheels do for smaller riders. Moreover, there could be some advantages taller rider could derive from learning on a larger wheel, namely being able to use longer cranks for better control of the unicycle. However, this is just my opinion, but I’d certainly like to see some academic study that explores links between relative height of the rider and the time needed for learning how to ride.
I could elaborate some more on this topic, I think, but I’m gonna leave it here and go practice. It is much more fun to learn unicycling than merely theorize about it.