does leg size effect movement points or is this just a myth?

I'm not really sure what you're asking here. Every character loses the exact same amount of movement points no matter where they are or what race they are, unless you have something working for or against you (hamstring, pathfinding, trepidation).

You get more movement points per level gain based on constitution though.

Do svirfs/gnomes run out of mv faster than races with bigger legs?

Magus answered. W/o external variables, all lose the same amt of mvment.