Folding Cycles and their Rider Weight Limits

Velocio explain it better than me;

It can be made fixed (fixed rear triangle), but as the wheel is smaller the flywheel effect is dramatically reduced and you have to put more effort into keeping moving. The dead spots on the pedals become more pronounced and whereas a large wheel would push the pedals through the dead spots the smaller wheels don't. It ends up feeling very weird.

So... can be made fixed, but doesn't make a good fixed.