isn't blowing out more about some moving parts that brushing against each other in regular wear. not really about stretching fit, I wear my jeans pretty tight on the top, but once the jeans shrink and stretch after initial breaking in, the size has become stable at my comfort size every movement afterward doesn't give any more stress to the denim than normal.
There are tons of theories out there for why blowouts occur. It does seem that some people are more prone to the crotch getting blown out than others. Personally it has never really been a problem for me. My worst offenders are my Strike Golds but even then that is a particularly set-in crease that has started to frey more than a traditional blowout. Your thighs rubbing together can of course damage the denim and cause a blowout in that fashion. Wearing a low rise jean and constantly putting pressure from walking can also seem to cause blowouts at the seams. There is then also the school of thought that infrequent washing is the main culprit due to set-in sweat causing the threads to become brittle. So basically no one really knows the reason but it does seem, from what I've observed, some people are more prone to it than others.