The muscle was there on that guy, just under the fat.  Those ads definitely show light/tanning/workout pump enhancements, but many of the dudes look like former college football players at 25% bf.  Many 30-something slobs who have let themselves slide still know how to train if they did in their teens/twenties.  If you work at a job where you use your arms every day, they will grow.  If you use your legs, they will grow.
  Again, I went from 235 lbs to 185 lbs in two months.  My jeans size went fron 36 waist to 28 waist.  I ran alot - and hardly slept.  I didn't use any gear, but the low bf illusion is always there - you look bigger when your leaner.