I've always gone the basic - Hulk's size and strength vary with his rage..
The madder = the bigger & badder..
And the pants..?
Can't we just look past that, it was always going to be a factor, you can't have the Hulk's massive green wang thwopping about in a battle or if he goes for a run, let the Emerald Giant keep his pants ON..
Maybe the Gamma radiation that he emits when he changes, stretches & alters the dye to make them huge and purple..?
but really, was anyone expecting no pants for him..?
does anyone want no pants on him..?
then drop it..