I really don't feel it's being rude at all. You OFFERED to accomodate yourself and haven't been pushing or complaining.

Even though you don't have much to lose, and are already "normal" sized, you're entitled to the same consideration I am when it comes to what you choose to eat. If they make it an issue after you've already given them an "out" they're either bending over backwards to make it easier, or they're trying to make it another type of issue...

As for the "c'mon, just a little bit", it's hard to say. People have emotional connections to celebrating with food, and some people just don't want to see anyone "left out" and can't understand anyone CHOOSING to be.