I haven’t been able to wear 501s comfortably since I was about 15. I got too big in the legs around 16 or 17, so putting them on was like trying to get out of a straightjacket. Also, to put it bluntly, my balls get squished, and I don’t tolerate it for the sake of fashion like I did when I was a kid. And yes, I’m sure the tightness is not because I’m too fat. My waist size — as measured by measuring tape, not flattery-prone clothes size — is less than an inch larger than when I was in high school.
There is a bit of size inflation, I think, but not all that much. I went jeans shopping a few months ago and found that with 501s if the waist is comfortably loose, the legs and seat are still uncomfortably tight. There’s also pretty wide variation in sizing. I always have to try on a few different pairs of nominally identically-sized jeans to get ones that fit right.
If they’re loose in the leg, and you’re certain they’re 501s, then maybe you’ve lost muscle relative to when you last bought some. If you have old 501s that fit differently, it’s possible that the cut is slightly different, but it’s also likely that they’ve stretched quite a bit with age. You might also just be mis-remembering how loose your old ones were.