Get a bigger engagement ring if you lose weight before the wedding?

Help settle an argument with me and my girlfriend. She thinks I am being insulting by bringing up her wedding ring size and what weight she should be by our wedding.

We have discussed getting married in the next year. I told her that I would get her the engagement ring we have looked at while shopping at Zales. She wants a two carat ring. I told her that if she dropped 30 lbs for our wedding, I would get it for her. 

She is 5"5" and about 160.

I also gave her a graduated scale of a quarter carat reduction for every 5 lbs she is off. For example, if she only loses 20 lbs, she will get a carat and a half sized ring.

She eats like a pig so I think she will be stuck with a half carat diamond when we get hitched. I actually took her to Olive Garden after we went to Zales and watched her load up on a truckload of all you can eat breadsticks. No way she hits neg 30 lbs! This will also be good since I cannot afford a 2 carat ring anyway.

She got really mad when I brought up my weight reduction goal for her. I think she is being ridiculous over this.

She thinks that she should get a half a carat reduction for every 10 lbs and not every 5 lbs. 

Who is right in this circumstance?