I am new to biking and these are my first pair of biking shorts. At the time of my first purchase I was 6' 0" and 185 to 190 lbs with a waist size of about 33.5 to 34, which has been fluctuating the past year or so. I ordered a pair of the large and they did stretch out on me noticeably after about 20 rides (2 of which were in a triathlon with a salt water swim).
I thought I had ordered a size down to my waist size and weight. As it turns out, I really could have gone smaller, probably because I lost some weight, so I bought two medium pairs (now at 175 to 180 lbs) and these fit better. I do believe my large pair did stretch out a little bit so I suspect the mediums will do the same.
Other than that, I have done multiple two hour bike rides with no complaints. These are comfortable bike shorts!