Gaining

say something like this
"Your belly looks so nice"
She mite grumble something about "I don't want to be fat"
then You say something like "I well love You no matter what size You are. but You mind If I have a little fun with Your fat be for You loose it"
or maybe You could try giving her a little belly rub.
or maybe try sweeping her off her feet.
BE ONIST with her!!!!!!!!!!!!!
and just bee your self.

Tell her straight up that she's looking good lately, and show that you appreciate her new look. Make it spontaneous though