I DO presume that the size is the only real difference, but if you want to know more differences, here are a few but they are indirectly related to the size issue.

Religions have more political power.
Religions have better marketing.
Most religions have longer histories of existence than cults.
Some religions have stories that are slightly less far-fetched than some of the more extreme cults. That has probably helped them a little in gaining followers vs. some extreme cults that have trouble persuading even the most vulnerable of potential members. However, if you ask a non-believer to evaluate the credibility of the stories of one random religion vs. one random cult, I don't think they'd find a nickel's worth of difference.