Firstly you would need to determine whether there is a correlation between race and IQ before you determine whether those differences are based on craniology or some other genetic factor.

Professor Rushton did exactly as you said. Check out his research that compares everything (race, IQ, brain size, and other achievement gaps).

https://www1.udel.edu/educ/gottfredson/30years/Rushton-Jensen30years.pdf

And if you're still not convinced for some reason, there's an even more modern paper that compares ancestry with intelligence. Those who self identified as having pure European blood had the highest intelligence. Those who considered themselves mixed (i.e Half Black/Half White) had intelligence in the middle range. And those who self identified as being completely Black, had the lowest intelligence.

https://www.mdpi.com/2624-8611/1/1/34/htm

It it's silly to assert something when you have nothing to base the claim on.

Human evolution is completely founded on the idea that skull shapes evolving paved the way for more intelligent species.

Again, look at the chart I first showed you.

https://i.imgur.com/qsca7Wd.jpg

It's logical to assume that a bigger cranial capacity is what gave rise to more complex thoughts and actions. For example, think about where language comes from. When babies are first born, they have trouble talking. But by the time they are 6 or 7 years old, they should be able to form complete sentences. WHY would that be the case if not for how their brain had been developing the entire time? And why can't we assume that a child learning to talk is indeed a genetic response that comes completely from the organ responsible for it?

If we understand this or accept this as truth, then you should understand why Blacks having smaller brains also plays a huge role in the intelligence gaps. They are half way between the Chimps and the modern evolved Man. Their brains are about as developed as perhaps a White 6 year old kid, who still needs a alot of help to survive.