It is absolutely not in any way like telling a black person to not be black. Being black is not in any way negative, apart from naked camouflage purposes in the snow I suppose (not a situation that would be encountered often), and also maybe more encountered the burdens and persecution society has come to lay upon some black people at some times, but anyway, being borderline obese is negative in many more ways (that actually exist compared to concepts society has dreamt up) than being black. Being too fat is not healthy, it is very bad actually, which is medical fact and scientifically backed. Being black is being just like you're coded to be by your genes, and not in a hugely important way unhealthy, maybe making vitamin D harder to get if you live far north. You have every reason and should use any motivation you have to stop being fat, and it is also very possible in the form of altering just what you eat and/or how much, and/or how much you move. You can't change being black in the same way as you can change being fat, and you also don't have as many reasons to want to.
Being just moderately fat, while not giving as big a motivations to do something about it, is objectively less attractive still, and compared to a black person stopping being black it is very very much more possible to not stay fat than to not stay black. Just eat different/less and/or move more as I said.
pic rel is girl who stopped being fat
I write a bit too much but I don't do anything too valuable with my time anyway