The reality is probably that kernel developers don't get any younger nowadays. And believe me, when you get older, have children and less free time, your waistline suffers a bit.
Or even a bit more than a bit.
A lot of people laugh at this, but it's painfully true.
Hell, I do Brazilian Jiu-jitsu three times a week, and lift/run on the other days. At a certain point for many, you just can't outrun a bad diet, and maintaining anything other than a dad bod requires all that exercise AND a strict diet. Good fucking luck doing all of that with a screaming baby at home!
I never thought I'd be fat, especially since back when I was 18 I could smash a meal at McDonald's and exercise right after, while keeping a six pack. Now, it requires double the exercise to not have tits...
I was in the same situation. I started to walk and longboard every day and track my calories without exception.
Eventually, you can see changes and it becomes easier over time, but oh boy I never shame anyone for their body size, because it's hella hard, and if you're coping with other problems like depression or addiction, it's gotta feel damn near impossible.
You don't need a lot, or even any exercise, to prevent overweight. Diet is everything. The amount of extra calories you can eat for a run is easily outweighed by the extra hunger.
Man I used to think there was no relation between what I ate and my wardrobe size, but eventually it really does catch up with you, especially if your day job is sedentary.
The one advantage of being significantly ill this year is that I can finally fit into large shirts and medium pants after years of XXL and XL of the former and latter.
I've technically gone down from XL. Seems L often fit me better now. But I've not changed.
Seems like sizes have shifted higher. Only evidence I have for this are souvenir t-shirts from 20 years ago that say XL. Which are roughly same size as L shirts I've bought recently.
I'm xxl but I'm also built like a forklift. When I was younger I could throw rolls of vinyl flooring over my shoulder. I wish I took better care of my knees though.
The shirt supplier may also be a factor. An American/EU size M is like an Asian L or XL. So if they changed the supplier then that's a possibility for the change.