Originally Posted by
AdelaaR
... they aren't allowed by the UCI.
In triathlon people use smaller wheels to gain aero advantage.
Actually if you use two 650 wheels it would be allowed by the UCI, the rule there is that both wheels have to be the same size (ie, you can't run a small front wheel wheel and larger rear one).
Most triathletes now run 700 wheels because they roll better, smaller riders (usually under 5'3" will use 650 because it allows them to get into a more aero position as the handlebars don't have to sit up as high.