How to ask her to do something without hurting her feelings?

I don't think you should say anything to her. Just don't date her. You shouldn't ask her to change her weight (and she really shouldn't change her weight just because of your preferences), just continue being friends with her and date a girl you're attracted to.

That would be like a girl telling a guy, "Yeah you're cute, but I hate the way you dress" Should he really throw out his whole wardrobe to suit that girl or should she look for a guy who's her type? Come on now

Actually The thing I can think of is you could ask her to exercise with you, but don't make it seem like Ok Sally you need to lose weight. Just casually invite her to the gym to play basketball with you.

Wait! I also thought of another thing! If she has a snacking problem. Bring in healthy snacks and share with her. If she's already full she's not gonna eat something else.
and I don't think its shallow I think its sweet you want to see this girl get healthier before you start a relationship.

I'd just be like, listen, I've been wanting to have an exercise weekend this whole month, but I kind of hate exercising without another person. Think you could help me by keeping me company? We'll be running and stuff but I promise it'll be fun. Something along those lines. If I were a little overweight I wouldn't get suspicious at an approach like that.

Sounds like she is a special girl. Yeah it is a little shallow but you may also care enough about her that you see her true potential - like being a healthier person. It is true true that when a person loses excess weight that they also build more confidence in themselves and become even more attractive. I think that you are at least being considerate of her true potential and not just knocking her down because she is overweight. So I wouldn't consider you to be that shallow. A lot of guys and girls have written about overweight girls and how most guys are so disgusted that they will back off and not be patient with the girl's struggle - which it is. Thanks for showing that there still are considerate guys out there who still find the inner beauty of a girl attractive enough that you would be willing to help her overcome other struggles.
I would agree with the girls below and try to find a social outlet for working out. I always enjoy physical activities when it involves a partner/friend that keeps it fun.
BTW, how much weight would you say she needs to lose to reach her potential? Does she ever show an interest in losing more weight and needing some help?

I would not tell her about the weight issue. It would hurt too much. Speaking for myself if my guy every said that to me. What I would do is try getting her to go more physically actives. Or even try to see if she would train to do a walk for breast cancer or other events. Make the physically active fun and not about the issue of her weight.

Note: I did want to know why is the weight the issue? It seems as if you are really into her. Why not just try getting to know her more for who she is not what she look like. Thing may just fall into place later on.

First, I'd say except her for who she is. If you can't do that now, don't go anywhere with this. However, if you are able to except her for her and you just think it would be fun to something together, suggest it. Swimming, biking, snowboarding, hoops, etc. Make it fun and do it together. As far as actually working out with the purpose to lose weight, not good. Unless she talks to you about helping her lose weight, leave her alone and love her for her! Good Luck:)

Tell her your got a new girl friend next time you see her - then add, "you know your cute, if you started working out, you'd be super hawt, anyways gotta run - cya around."

I don't think that there is any easy way to tell her, if you say it to her face she is probably going to be upset.

But at the same time, just getting her to do some activities most likely isn't going to make her lose substantial weight. So if that really is the only thing holding you back from a relationship with her I guess you might as well tell her unless you just want to keep her as a friend. Just try and keep it as unoffensive as possible.