it will stretch even if i wear them a little looser and have to wear a belt? won't they shrink even more after the first wash?
well the waist (and knees) stretch a lot because the is a "push pull motion" and constant movement in those areas. When you walk your ass, hips, and upper thighs pull and move the denim around, and even though they are not necessarily tight, the fabric is still being stretched and pulled in those areas. it will add less than an inch but definitely something noticeable. With enough time, it takes a very little amount of tension to stretch cotton fibers.
for your second question, the denim does shrink more relative to how much youve stretched it but I believe (and someone correct me if im wrong) that the measurements will be the same post soak. For instance, lets say your knee measurement goes from 7" to 7.5" back to 7.1" after washing. But on the same pair there is less tension for some reason, and it only goes from 7.1"-7.3" back to 7.1". Now technically there is more shrinkage but the post wash measurement should be the same