The phrase "smaller skulls" means that the size of people's heads or bony structures around their brain is relatively smaller compared to others. Full definition
She was believed to be female because she had a relatively small skull and small canine teeth. (sciencenewsforstudents.org)
It's a surprisingly small skull, but it shows some intriguing similarities to humans. (discovermagazine.com)
Most domesticated mammals, including dogs, tend to have smaller bodies than their wild counterparts, with smaller skulls that have shorter, wider snouts and shorter, lower jaws. (sciencenews.org)