Because, generally speaking, women have a slightly different view of things. We tell you boys over and over again, if you love her and want to be with her, and want to father children with her, realize her body WILL change. If you spend more than 20 years with her even if you are childless, her body WILL change. Gravity catches us sooner and it's meaner to us.
So you're being critical of your girlfriend of 9 months for gaining 10 or so pounds tells us a few things:
1. You really aren't in love her.
2. You won't stay with her.
3. Your relationship with her is heavily focused on physicality/appearances.
You're asking us why women who get comfortable in relationships gain weight. I think she's, subconsciously, hoping you'll leave so she can hook up with a guy that really wants to build a future with her instead of hanging onto you when you're clearly not that guy.
You asked