Yep, it bugs me, but I'm getting used to it, and as DS gets older, we hear it less often. To me, it always felt like a reminder of how I failed, and it hurt. He's still small, but most people don't know how big a typical 8 month old is so they don't notice his size as much (that, and his gigantic head seems to compensate for his small size!). When he was super tiny, my friends made him a shirt that said "I may be small, but I'm big where it counts" and another that said, "please don't tell me how small I am, or you may give me a Napoleon complex".
Honestly, what irks me most is when good-willed people say, "Oh so-and-so had a preemie 20 years ago and he weighed a pound and now he's 6 feet tall and 275 pounds". Sheesh. I hear that soooooo often. I know they just think they're comforting me, so their hearts are in the right place. No biggie.
"In the depth of winter, I finally learned that within me there lay an invincible summer"