Re: "He's so skinny!" and "He's so tiny!"
My daughter was just a little over 5lbs when she was born a week early. I was diagnosed with rH disease AND IUGR while pregnant with her, my pediatrician wrote "poor growth" on her chart and kept strongly suggesting I supplement. I felt HORRIBLE! Even though doctors are there for a reason, you can't always take their advice - they go by charts which map an average and don't necessarily apply to an individual. My daughter is thriving and healthy and has always been small (but TALL) for her age.
On of my sister-in-law's sons is really skinny - it looks like his pants are always falling off! I remember she'd try to hide his figure in baggy clothes because our mother-in-law would harp on his size, say he wasn't being fed enough, etc. His father is long and lean and I think he just follows suit.
When it's all said and done, you're the mother - follow your instincts. Is your baby healthy, happy, thriving? Then it's all good.