They’re selfish jackasses. They think “oh that lady has a bit of weight around the middle, maybe she’s pregnant, how fun would it be if I’m right and I get to congratulate her! That will be so fun for us both!” And it doesn’t go deeper than that. They want to be seen as caring, observant and intuitive and miss the mark on every single one.
I’ve been asked this a couple times. My reply, “nope just fat, I’d appreciate it if you left me alone.” If they try to apologize, “the best thing you can do is stop talking to me.” No telling them it’s OK or not a big deal, do not accept any apologies. If they keep pushing it, fuck off is totally appropriate. These people deserve no kindness.