Rather than tell her that her weight is an issue for you, perhaps ask her how she feels about her size, without putting forward your own opinion/concerns first. You could maybe say..
"Stop me if this is an area that's difficult to discuss, but i've been wondering how confident you feel about your appearance? Does your weight ever affect your self-esteem or are you comfortable with your size? I'd hate to think that you have struggled emotionally with being a bit bigger, because I really care about you and think you're awesome......"
She will either tell you:
* that she's made peace with being overweight/obese and has come to accept her body this way. If that's the case, stop seeing her if you can't get past it so she can find somebody who will. She shouldn't have to change for anybody.
* or... She might spill her guts about her weight struggles and confide that deep down she wants to lose the excess weight but might not know how to go about it. It would be daunting and very scary for somebody in this position. But if this is what she wants, you can open up a dialogue for discussing how losing weight could be positive for both her health and also for your relationship.