How would you feel if your waifu gained some weight?
Wouldn't really matter to me either way. I dig girls of all shapes and sizes.
She's very active physically, what with her job as a mecha mechanic. The weight would go to all the best places, and she'd burn off the fat fairly quickly.
I'd nicely encourage her to work out a bit and even go as far as cooking healthy, nutritious meals for her everyday.