weight gain and kittens

Jewel and George were born in early March this year. All vaccines are up to date. They are basically healthy. They were spayed and neutered respectively in late Sept. Jewel being the female has always been smaller and up until the surgery was about 7 lbs to George's 8lbs. Lately, my fiancee and I have noted a weight difference between them. She's heavier and her abdomen is rounded more than before. She's still pretty active and she's eating and using the litter box ok. Her incision healed completely and uneventfully. Should I be worried?

I'm certainly no expert but don't think I'd be worried as long as she is eating, drinking & using her litter box regularly.
I wouldn't be surprised if although she is active she is just not putting out the energey that she did before her surgery. That't the way my Mandy did. Her incison healed fine but she just didn't do the almost frenzied play that she did before. She acts more like a sedate 'old lady' cat now. She CAN sometimes act up like a kitten but it takes more effort to get her interested in doing so.
Her best form of exercise now is chasing the red 'bug' from the laser light toy. She will go crazy over that. She comes running with her eyes big & bright & all excited when she hears me take it out of the cupboard where I keep it.
If you think your little girl needs more exercise I would recommend trying one of these laser lights.
Best wishe & hugs to you & your kitty babes.

Mandy