The anal fin look pretty pointed to me so this platy might not be a female after all. Young males can tank some time before developing their man parts and will look like females till then. I might be wrong, but you might find that the platy's anal fin become more pointed after a couple more weeks.
The fish doesn't look all that fat, but rather a healthy weight.
If it is a female, she doesn't look all that pregnant but you should assume that all females that were kept with males are pregnant.