Maybe, but she's never been close to "fat"ever and now she's getting a little bit...puffy. I'm thinking of mixing it with a non-fish flavor or something and seeing if that works, b/c I hate to take something she enjoys away from her but good lord is this a pain in the butt.
As far as her weight, I wouldn't react too quickly. I don't know how long it's been since she changed but, if she wasn't a fan before, she'd be eating just enough to satisfy herself. If she now likes it, she'll do a bit of pleasure eating. But, if it's dry food, more food means more water, just because
it's dry. There's always a bit of fluctuation when there's a change of diet, which is much more noticeable on a svelte feline, rather than a tubby tabby. But,, if you haven't increased her calories available from food significantly, then it tends to work itself out. Her activity level should go up accordingly, which also makes for a more annoying animal.
Mixing it isn't a bad idea. I'm just saying don't worry too much about it, yet. She's still young, right? If so, she'll be able to adjust in a change of diet pretty well, usually.
Good luck.