I have been using Pearl Izumi shorts for years, and always size medium, and I have not gained or lost any weight.
These shorts fit like a size large-really a little too large, but I washed and wore them, so now I suppose I am stuck with them.
From now on, I guess I'll have to guy my shorts at a shop and try them on first.
I've been using the same bike shorts for about six years, and thought that bike shorts were uncomfortable by their nature. This pair of shorts is so much better than what I was used to! I'm very happy with my purchase.