Tiger barbs are egg layers, and the females are usually larger and rounder with an all black dorsal fin. They reach sexual maturity when they around 1 inch in length, so that may be the explanation for the change. Tiger barbs also tend to be pretty greedy eaters, and will usually fatten up quickly in a community aquarium. Do keep an eye out for any behavior changes, sluggishness, or clamped fins which are indicators of illness (like dropsy).