How old is she? How heavy is she?
I know Syrian sizes vary wildly -- from less than 100g for small pet shop Syrians to Lougarry's guinea-ham litter, who are almost all over 200g and still growing
If she seems happy and healthy in all other ways, I'd be inclined to assume she is just a small ham
(Plus size doesn't necessarily correlate to weight -- my Annie put on weight the first few weeks I had her but didn't look bigger, and hasn't put on more weight recently but definitely feels chunkier since I changed her mix a few weeks ago.