I know it is hard when you see their sweet little faces not to give them a treat. I give Nikko treats for merely breathing!!
I haven't had him neutered yet but they say they tend to gain weight after they are fixed. I am sure when the weather gets warmer you will take Lexi for walks and she will start to lose weight. But like someone else said, if she was really in some danger your vet would have let you know. So don't worry so much and keep being the great and devoted mommy you are.
And if all else fails, we can keep blaming grandma.
(j/k)