Been there, done that. I started scrawny and the first thing to get big was my ass. Girlfriend noticed and always laughed when i was standing without a shirt on side-on to her view (I'm a little lordotic, so it exaggerates even more). Then i got a comment from my buddy's girlfriend "holy crap, dude, your ass is getting huge" and then one from a co-worker when she was walking behind me "I'm not checking you out or anything but I didn't believe you when you said you gained 30lbs, now i see it".
Pants shopping is useless. I went to West49 (baggy skater clothes) and I tried on 9 pairs of "relaxed fit" jeans. I couldn't even get my legs INTO 5 of them and they all had a waist that was 2-4 inches larger than my waist.