Ah, you're thinking of this:
See, the problem with that is, you've never seen Mrs. Cake from directly in front before. It's always been from an angle, where she's more obviously chubby. I went back and watched every appearance of her in previous episodes, and I couldn't find a single time when she's shown directly from the front like she is above. You've never seen her like that before, therefore she seems different.
But if you look at her in the new episode when she's shown from any other angle than directly in front, she still looks exactly as chubby as always.
Also, here's Pinkie:
Compare closely with Mrs. Cake above. Pinkie's face (and that of the other mane 6 and most other ponies) is slightly more elongated than Mrs. Cakes, neither is a perfect circle but Mrs. Cake comes quite a bit closer to that.
Look how much of Pinkie's face is outside the circle compared to Mrs. Cake. Also for some reason her ears are way lower than Pinkie's, though I'm not sure what that means
Conclusion: She's still chubby, you've just never seen her from the front. At least I couldn't find an example where she's shown from the front, if you can find one I'll be happy to compare them. But again, from any other angle she still looks just as chubby as always and really, the place she'd lose most of her size should logically be the tummy if you were right.