You are not alone.
I am the same way. I am constantly thinking about my dd's size and it affects me emotionally etc....I see these babies her age and even younger with size and I get jealous, I do. It sounds stupid it really does. But I wish sometimes that she was of normal size.
I get ppl asking me how old she is and then they are shocked and dont believe me and I have to whip out my BC just to prove her age and so on.
But on the other hand I also love it at times, makes me think she is still a baby and not a toddler. But her not walking or talking yet gives me more of the (she is still a tiny baby) feeling all the time and it makes me sad.
My dd is 13 months old, weighs 16lbs and is only 26 1/2 inches long...picture this, a 13 month old the size of an average 5-6 month old, and you have my daughter. Size 3-6 months clothes and sometimes 6-9 (brand depending) and not even a size 1 shoe yet and wears premie socks and slippers.
My dd has never been failure to thrive though, she is in the 3rd percentile.