My boy is average size, and these fit him just fine.  I bought him the size he is growing into next basically since he can go either way.  The shirt is long enough to hang over the top of the underoos a few inches which is good for a wild sleeper (not exposing his belly all night).  We are very pleased with the soft cotton.  The shirt is not too thick, but not so thin and flimsy that I'm worried about holes soon.  It is a good lightweight for summertime sleeping!  The underoos are thicker fabric and also suitable for nighttime shorts. Happy customer!  I found the same brand has Spiderman, so we bought that also.  He loves them.