...I can relate because my family was seriously dysfunctional - I have a lot of compassion for your situation...
NyaChan, you are right. Your grandmother is toxic. And those in the family who allow her to treat you and others that way are enablers. Kudos to you for seeing this at a college-age. I was several years out of college before I learned that this was not normal behavior and I didn't have to put up with it and "respect my elders" by being treated like a doormat.
You behaved impeccably. You were very patient and classy. The only advice I could add is to avoid toxic grandma like the plague in the future.
First of all, 5-6lbs is nothing. I hoped not to gain the freshman 15 my first semester of college. But my mother is very very much like your grandmother. I starved myself in high school and with my lousy metabolism, I was still only in just a healthy weight range, not thin. Without my mother around to badger me in college, I gained 30 lbs the first semester.
Second, personally, I was in college to get an education. Sure, it would have been nice to have been healthy too. But I was there for a degree, as I assume you are too. No one can be perfect in all areas of life. To me, spending time with my books was more important than spending time at the gym.
But regardless of the topic, your grandmother is rude, mean, condescending and toxic. And if you had lost 5lbs, you would have been mocked for being too thin. Or grandmother would have mocked your hairstyle, haircolor, clothing, height of your forehead, how far your ears stick out, how much or how little you talk... you get the point.
Hindsight is 20/20. I was in your shoes at your age with a similar family. Now that I'm in my mid-30s, I have given the cut direct to most of my family members because many of them are like your grandmother. Most of the rest are doormats and enablers who just want to keep the other doormats in line so they're not the only ones being targeted for abuse.
I say you should refuse to go to any gatherings that toxic grandmother is at in the future. The only way I'd go at all would be if your parents are paying for your schooling and threaten to kick you out or not pay tuition. And in those cases, I'd drive separately, and I'd leave the event altogether the instant toxic grandmother started being passive-aggressive. Or if you have to drive together, bring a good book and return to the car and sit there until the others are ready to leave.
And because I've heard from the other enablers and know what they'll say, I'll tell you in advance, you refusing to put up with grandmother's abuse - and that's what it is: abuse - is not
"too sensitive" or an "overreaction."