To answer the question based on the picture, I'd say if she is pregnant she isn't very far along, ans based on the general live bearer pregnancy cycle of 28 days I'd say she's in her first week, possibly early second.
Guppies are like all other animals that bare their young, they remain (Rather) small until about half to three quarters of the way through their pregnancy and then in the last part balloon up as the babies pack on weight and prepare to be born.