Well, let's look at the facts.
*quickly makes some research on the web*
It seems a modern human brain has a volume of 1,600 cm3.
1,600 / 3 = 533,333333 etc...
What we know is that the "original size" mentioned by the Terra Tower stoneface must be of roughly 530 cm3. According to many websites, the brain size of an Australopithecus is about 530 cm3 (!). This is too close to be just a coincidence for me, but I think there's a blurry zone: in French, the term "lignée humaine" refers to all species from the Australopithecus and the Homo families. In other words, calling an Australopithecus human is perfectly possible in French (obviously it's not always the case, sometimes it's just the Homo species, sometimes just the Homo sapiens, but it's still possible and not shocking). I don't know about English customs though.
So, if you don't want to consider the Australopithecus human, there's the Homo habilis who has a brain size of 550 to 650 cm3. It's less close to the 533,33... but it can fit.
Anyway my point is that, when the stoneface says that a modern human brain is 3 times bigger than the original brain, I believe it was comparing the brain size of the "corrupted" modern human with the brain size of either the Australopithecus, OR the Homo habilis, OR some other early human species. It couldn't have been referring to a more evolved species like the Homo erectus, because erectus already has a brain size of 1,000 cm3... too big to be 3 times smaller than the 1,600.
As for Ayla, she's clearly not an Australopithecus, she might be erectus or someone with no equivalent species in the real world. Anyway, her brain size is between the "original size" mentioned by the stoneface and the modern, 3-time bigger size. Her head is clearly too big to contain just a little 530 cm3 brain.