Jesus, the reason women seem to be so sensitive is men have made a point to make talking about these things as awkward as possible by not being honest.
Every which way you could say this in a round about way is telling her you both can't be honest, she'll know you're trying to find a nice way to call her fat anyway and will lead to long term communication issues.
Put it this way, if you go the round about way of saying this, she'll be angry and (if you love her to bits and eventually want to marry/have kids) are setting yourself up for a lifetime of awkward conversations, dishonesty, distrust and generally being unhappy. Tell her you worry about her weight, that you love her and its hard to think about a future with her and kids if she is unhealthy.
For me, with parents who taught me to eat very poorly, contributed to me being very unhealthy, it's simple. I want to eventually have a family with someone who knows how to eat healthily, will pass on those traits to my children and will help my children be healthy and fit, particularly throughout their childhood which for me was particularly unhappy due to ill health and weight problems.
For me if the woman I'm with can't understand that, she's not someone I want a family with. Men need to get to the point with these major issues because you end up thinking something will change.... and it won't. In 15 years married with a kid being fed McD's you'll be angry and resentful and maybe/probably eventually split.
You know this is a serious issue, treat it as such. There would be less divorce and less unhappy marriages if people were just upfront about what is important to them.
If you might want to marry her, how important is this issue to you. Will you resent and be angry at her for being overweight and doing nothing about it, or does it not matter to you, if the former, its a damn sight nicer to just be open about it now then resent her, waste a decade of her life and end up divorced and bitter isn't it?
There is nothing wrong with you laying your cards out and saying "I'm looking for A, B and C", you find out you don't want the same things and move on, it's better for everyone.
