Small, nervous male maine coon kitten?

I bought two pedigree maine coon kittens this week from a reputable breeder. They are 4 months old brother and sister, neutered. I have had them for 2 days. The female is an absolute doll- into everything and very chatty, but the male is very timid. He will let me pick him up and he will sit on my lap, but as soon as he's on the floor he will skulk off (crouched low, tail down) and hide somewhere. He has not eaten much from me and is quite skinny. He is a lot smaller than the female. Should I be worried because he's very nervous? Why is he so tiny compared to his sister of the same age? Will he grow into a big maine coon one day? (- his parents are medium sized)