I can't belive the amount of people on here slating what henry's saying.
Let me guess, you're all single and / or fat?!
It's not healthy to be obese and if my girlfriend went up 2 dress sizes in just over six months i'd be concerned too!
Physical attraction is a big part in a relationship. It's the first thing that you notice about that person and draws you to them!
Anyway, i could rant all day at those ignorant people but he's some constuctive advice...
Try spending weekends doing things rather then in the week. Like Swimming (even if it's a water park for starters). Try walks in the park with a stop off for a healthy lunch or maybe a drink. You could also get something for the house like Wii fit or a dance mat, something that makes exercising fun. Because to be honest, i don't find exercising in the gym fun either but me and my girlfriend do those things and it works.
Also, try talking to her, there maybe something emotional that's bothering her.
I hope that helps, and just ignore the majority of feminists that replied to you.