One, she shouldn't lose weight "for you", if she wants to then that's her perogative. If you don't find your girlfriend sexy anymore because she's put on 10 pounds or something, well you're a scumbag.
But if you've got a good comfortable relationship, well sure you could point out stuff like noticing weight gain, but done in a nice way, and certainly not pressuring them or making them feel bad.
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One Piece: Pirates with style!
-= Metal Gear Solid: Tactical Espionage Action =-