Lots of advice re reducing and preventing the appearance of stretch marks on RJ, do a forum search in addition to reading people's comments here.
As to why you get them and other bigger guys don't, it's mostly genetic. Despite being "ancient" and undergoing a dramatic 80lb weight gain/loss I not only never got them, I have hardly any loose skin and the little I finally developed 'round my belly since I cut to the point of having a visible six pack will, I'm told, snap back into place within a year because I have "beautiful, elastic skin that never got stretch marks."
Lots of big young guys at the gym appear to wear their stretch marks with pride, reveling in them as a sure sign of growth. If none of the advice on eliminating the appearance of stretch marks works you may as well make the best of it!