How much of a part does rider weight play in knee down?  Quite a bit, actually.  Aside from the obvious things like weight distribution and placing more weight on one side of the bike, larger riders are more likely to struggle due to the positions knee down requires them to put their body through.

Obviously, this depends on how overweight the rider is and their general level of fitness.  Hanging off the bike requires you to use muscles you wouldn't use in everyday life.  As larger people have more weight to shift around, it puts more of a strain on these muscles than lighter people, resulting in exhaustion much earlier than a fitter person.  Heavier breathing is also likely as the pulse starts to race with the adrenalin pumping, which could consequently result in an increased likelihood of the visor misting up.

In summary, it's perfectly possible to get your knee down if you're overweight, but it's probably a lot easier if you're not.

This article was written by an overweight bloke, so trust me, I know! ;)