You could tell her you'd ask her out if she weren't so FAT that it physically repulses you.
You don't need to be so blunt about it. although it might be exactly what she needs to stop eating all those pizzas, pies, chips, crisps, ice cream, chocolates, twinkies, doughnuts, pies (have I already said that?), burgers, steaks, bacon sandwiches, baked beens, pancakes, cakes, hot dogs, lard....
She might lose a butt load of weight for you.
Or you could just get on with your life and not compromise on size. You'll find someone else you get along really well with who is more physically attractive to you.
I've dated a few chubby girls though, and they've all been really excellent girlfriends and lots of fun.
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