She's bigger than the other platy I have with her and the other one doesn't have the dark spots near her rear belly so I'm presuming shes pregnant. I got them both 3 weeks ago and I noticed she was quite bigger than the other instantly.
If she is kept with a male then she is most likely pregnant. And she looks pregnant too. If she isn't with a male now, she probably got pregnant at the store you got her at, and can keep the sperm in her body for a very long time.