From what I understand, this kind of phenomenon is called "vanity sizing." Usually seen more in women's clothing but I suppose men's clothing isn't immune either. Supposedly over the course of time clothing companies have started making clothing bigger and bigger with smaller corresponding size numbers. For example, what used to be a size 12 might be considered a size 8 now etc etc. This is probably the case with your pants. The tag may say 34, but the actual measurements are much bigger, as you have found out.