There is NO way to do this.
You can be absolutely sure she knows about it. Has she said anything? Maybe she's afraid to talk about it because she thinks you're turned off - and she'd be right, as it happens.
It sounds like she's not too happy about your reaction and she's just kinda waiting for you to say something stupid, so that she can tell you where to go.
Look, if you like her, you like her. If you only like her because her body is a certain way, then tell her that so she can stop wasting her time with you and find a real relationship.
One thing to think about, why is she getting fatter? A new job where she can't work out as much? Change in diet? Is she depressed? Any medical problems? Heredity? Stupidity?
If you can get past the "you are too porky" part and figure out with her what is causing it (nicely), you may have a chance.
Otherwise, your only hope is to continually drag her to things that require extreme fitness - triathlons, mountain climbing, kayaking, and so on, and not let her get away with sitting out. She'll get fitter really fast, if she survives.
Good Luck.