Whenever people make comments to me, I usually make a joke out of it, thank them, or say, "I'm taking that as a compliment!" (That last one is my favorite and my go-to.) (Also, I am a bit eccentric, especially at my stuffy job, so I'm no stranger to receiving comments that some may take as rude.)
I no longer remember percentages, but a really high percent (like 40%) of our happiness is based on how we react. You can get offended, make a snarky remark, and make people walk on eggshells around you, or you can take a step back and not let it bother you. (I'm not talking about romantic or other deep relationships, but more superficial ones, like this one sounds to be, or like the ones with my coworkers are.)
I always assume good intentions unless there is reason to believe otherwise. Given how you described yourself, I doubt this person was serious. If you were overweight, then I'd agree that it's really rude.
Since you are thin, my guess is this person is a bit socially awkward and made a (bad) joke. I am very athletic, in great shape, and eat about 3 times more than you'd expect for someone my size. People always remark on it, and I'll just joke about winning eating competitions, picking up a second job to pay my grocery bill, or saying that's why I run so much -- my cookie addiction, etc.
If someone makes a comment about your clothes, respond, "Thanks; I love these pants!" or "Isn't it so pretty? Thanks for the compliment!"
I'm sure there are people who will disagree with me (there are already above me!) but I feel that life is way too short to let people like that get you down, and deflecting usually does the job.
Now, if it's a really rude comment, I'd look at them and say, "Why would you say that?" but I've only ever had to use that with XMIL!!!
ETA: To make it clear that I receive and don't give odd comments to people.
[This message edited by phmh at 10:14 PM, January 2nd (Thursday)]