My kids have smallish head circumferences too.. older DD is probably in the average range of intelligence, younger DD is gifted.
My parents are visiting (they live 3,000 miles away so we see them about twice per year) and the very first day my mom read a book to DD who is now 26 months. DD identified every single animal, which she has been doing for so long I forgot that most kids that age aren't quite there. My mother's jaw dropped open. She has been substitute teaching elementary school lately, including preschool, so she has some basis for comparison (she is a retired HS english teacher). She also spends time with my cousin's kids who are around the same age. She was also really impressed with younger DD's jigsaw puzzle abilities. She likes to ask for help, but she really just wants you to hand her the pieces so she can figure out where they go. She does 50 piece jigsaws right now, with minimal help from us (it helps if we find the group of pieces that all make up one part of the puzzle- like the sky or the flowers, and then she figures out how to put them together). My 4 year old is about the same with jigsaws, and she actually gets more frustrated and gives up more easily while doing them.
I'm still so impressed by my daughter's vocabulary. She knows all the words for the different fingers (pinky, thumb, ring-finger, etc). Not a huge big deal but it's always fun to find different things that she knows. (I discovered this when she screamed "I hurt my pinky on the cabinet!" yesterday).
One funny thing is that younger DD goes to preschool with older DD, and as far as I can tell she is pretty silent at school so the teachers have little clue how advanced she is. They do note that she is very curious and seems to always be paying attention. There are a couple of other 2 year olds in the group (it's a multi-age class, 2-5) who usually just wander over to the toys when the older kids are doing a group activity, but not my DD- she is always engaged, though quietly. Anyway last week I told them that younger DD sings all the songs from school at home (they are in hebrew- it's a hebrew immersion preschool), and the teachers thought I meant older DD (who is also on the quiet side, but she does speak at school this year, in both hebrew and english- last year she was 3 and she never spoke). I didn't bother to correct them, what's the point? They get that the kid is smart, but they have no idea :). She's happy & challenged in school, so that's all that really matters.
I do wonder what it's going to be like as younger DD gets older. She appears to be advanced in everything she does- she's very athletic though I have seen stronger & more coordinated kids her age. It's impossible to know how it will all unfold, so it's exciting. I'm also excited about older DD.. she is also talented but in different ways. Everywhere she goes she makes friends, and parents always tell me how their kids talk about her- she doesn't seem to want popularity, it's more that she is naturally interested in other kids and wants everything to be fun all the time for everyone, and people are drawn to that. She is very shy with grownups though, so it's an interesting combination. I was shy with everyone as a kid, so it's so strange and different to me. And my husband is in some ways more outgoing but he is somewhat socially inept (he's super smart and that is part of his problem- he comes across as arrogant and strange sometimes).