Hi, she doesn't look too big to me. I find platies to be round anyway. If she was pregnant I would say she's only at the very start of the pregnancy . She doesn't have a gravid spot but that isn't always an indication of pregnancy anyway. You are best going by shape and size.
This is one of my females, I am using her as an example because she dropped fry almost a week ago and as you can see she is about the same size, if not bigger than yours, yours may be pregnant if she was in a tank with males.
This was her days before giving birth-can you see the difference? it's hard as she didn't have a gravid spot either. Typically they square off but mines didn't do this either, her belly just hung.
This is why I am guessing that your female might not be pregnant (usually platies and guppies come pregnant from the LFS) if she is, she still has 3-4 weeks to go.
She's very pretty, I got one just like her only today! Now I know she's a sunset platy