Well really at the moment the weight classes that have mass appeal are..

AntWeight (150g) FeatherWeight (12Kg) HeavyWeight (100Kg)
There is big gaps between them all but the other less publicised classes are ....

FleaWeight (75g)
Super AntWeight (250g ?)
Kilobot (1Kg)
LightWeights
MiddleWeights

Chris - http://www.featherweights.org/forum