I have only had one butt really stall - I mean for more than an hour or so. I buy the ones at Sam's and they are usually in the 8-10 lbs range. However, I told a neighbor that I was cranking up the smoker a few weeks back and invited him to throw something on there if he wanted. He went to Publix and brought back a 5 lbs butt and picnic that was somewhere just under 5 lbs.
Did my usual process and temp probed the 5 lb butt - figuring that it would be done first. Nope. Even foiled it stalled and took forever to get past 170. My two 9 lbs butts were at 208 and 210 by the time the 5 lb butt reached 195.
Maybe JJ is right about younger, smaller pigs.