I have never once stated I don't respect fat people. I have no issue with anyone living their life the way they choose.
You certainly imply it with the below comment, which could be construed as as a thinly veiled representation of your true feelings on the subject hidden behind your politically correct reply
Maybe not just eating certain foods but when people let themselves get obese and bigger I think we can all agree that they lack much self respect. Yes it does differ in us all how we judge ourselves but I think weight is something everyone should think about controlling.
But I digress. OP - If she was the same size and had the same eating habits when you met, then what you are effectively saying is that you want her to change. That is not really a strong starting point for a relationship.
However, if it does concern you a lot - I would just be honest with her about it. No need to be rude or mean, just tell her how you feel. Lets face it, if you cant be honest with each other then you have foundations of quicksand.
Yes, it may hurt her feelings - but she needs to know how and why you feel the way you do. Accept that this relationship may not be right and you may go your separate ways.
Holding onto this paints you as someone who is a little insecure yourself. If it is not right, it is not right. Tell her why you think so - if it does not change anything then you are not meant to be. Move on.
Otherwise you will get stuck in a rut with someone you are not attracted to, but through tragic irony you will not tell her something to save hurting her feelings and yet by not doing so you are allowing her to hurt herself far more in the long term and all the time you are unhappy and resentful of your own weakness in confronting this issue. At some point, something will give. Best to get out sooner rather than later.
If she is genuinely happy in her own skin and there are no underlying problems with depression and insecurity that you could both work on, then I think it is you who needs to make the change. You need to move on because clearly this woman is not right for you and never will be.