the shorts should be tight, but not so tight you can barely squeeze into them and certainly they shouldn't cut off your circulation. you don't want them loose, because when you get up off your seat they spare material can hook onto your seat (has happened mountain biking, not so fun). When i try mine on, i see if there are any loose parts bunching, and i give the shorts a little tug in spots, and see that they "bounce back" so to speak. some shorts are less stretchy than others. i try to avoid those, since a friend bought a pair and now they are HUGE on him. i mean like boxer shorts. I have never had any shorts shrink, what you buy is what you get.
good luck
Han