I'm a girl so I guess I can share my opinion with you.
Well, first of all, it would hurt her feelings if she finds out that you don't want to be married to a "tatted whale." If you don't want to be with someone who is a little bit on the voluptuous side, then don't. I personally find it stupid when men care about looks above anything.
Anyway, this can only go two ways - either you end things with her (because she's getting bigger, which is pretty shallow, BTW), or you motivate her to start a healthier lifestyle. And since you mentioned that she's turned into a party girl, I suppose that meant she's been doing a lot of drinking which is generally unhealthy. Talk her out of it, and take her to the gym with you. Take working out as a form of bonding for you two, get creative with it and make it something that you two would enjoy doing together. It won't just help in making your girlfriend healthier, eventually, it can lead to a healthier relationship between the two of you as well.
So.. good luck. Whatever decision you two come up with, I hope you both meet at mutual terms.