It's happened to me, I've been in a flux between overweight and in shape most of my life. I wrote a really long comment one time on ways I noticed it. I feel like I should repost parts of it, if that's okay. I think you may have noticed similar things, OP.
When you are overweight people will assume that you are less intelligent and frequently interrupt/try to discredit what you are saying in conversation. Oftentimes, I would say something relevant just to be straight up ignored. When I got in shape, people gave me the floor and didn't challenge me nearly as much. I'm usually listened to now.
While overweight, people were way more apt to be rude and demeaning to me as a form of humor. I noticed this among friends. People would just feel totally okay with randomly insulting me/ a comment I made to get a laugh out of the group. Now, this rarely happens. If it does, a disapproving from me look doesn't merit further demeaning/someone telling me to learn to take a joke like it used to. Instead it means that people will take my side and stand up for me or the person who made the comment will quickly apologize.
People would automatically assume I was nice/a pushover when overweight. Now people automatically assume I'm more confident/more likely to call them out. If I called them out, I couldn't take a joke. Now, people at least don't automatically assume that I'm okay with whatever shitty thing they are saying. If I call them out, it more often merits an apology as opposed to an insult.
I could blend into the background so well. Seriously, if I didn't want to be noticed I didn't have to be. It's not really the same.
I got way less male attention, but the attention I did get was way less aggressive than the attention I get now. While being overweight, people would compliment me on the fact that I'm interesting and try to engage me in conversation to get to know me as a person. Now? People walk up, stare at my tits and ask me what size they are. Frequently. Or walk up, buy me a drink without asking and demand my attention. Or hand me their hotel key and wink, telling me we'll need it later. Or just slap my ass out of the blue. okay assholes