I didn't notice any difference in speed or warp time between harvesters and small harvesters so I create small harvesters until I skill up to regular ones, and then i slowly replace the small ones I've made.

If the only difference is cargo space, it's clear you should only make harvesters. Also the amount of cargo they hold is so much more than a small harvester that I think it's a moot point to even bother with small harvesters after a while.