A.F.
Your little girl seems very bright! I don't know if this applies but when my daughter was in the womb they told me that her head was small and basically put me through hell worrying about her and she is perfectly fine.
Also, have them do several head measurements. Sometimes you get a nurse who couldn't measure a ruler accurately:) With my son , the Dr. was freaking out because his head was "too big" according the measurement. He wanted to run tests and I asked for another measurement and it was perfectly normal. Plus, every kid is different!
It's hard not to worry but I think the proof lies with your daughter. By any measurement she is smart, active, and sounds like she is a delight to you and that will never change!
Definitely do the tests if the Dr. recommends and take it as it comes. Don't borrow worry. Most likely she is just made small like you said!
Blessings on you!