Princess certainly sounds as if she is overweight. This has nothing to do with spaying, but everything to do with what she eats and how much she moves around.
There are charts on the web showing proper cat silhouette but the best guide is the naked eye.
Stand over Princess and look down at her while she is standing up. If she is shaped like a football, she is overweight.
You should see a marked indentation just before the hipbone (that is the waist) and her pelvis should taper toward the tail.
If you look at her from the side her belly should tuck up a bit at the back. (Gizmo's tuck is only visible when she stands on her hind legs since she has a bit of droopy skin there, so I go by the 'top view'.)
If Princess is a Maine Coon cat she will weigh more than most other breeds.
Gizmo was actually a pound overweight this year and she lost the excess water when I put her on non-allergenic food (rabbit meat instead of chicken.)
If you are feeding Princess a store brand like Meow Mix or Fancy Feast, these are mostly filler. Get a good brand of cat food that contains no corn or wheat or byproducts and has a high meat content and her weight should come down. Play with her more to get her to exercise and run around.