That means the chain of thought “brains volume decreased, so individuals must have gotten less intelligent. Yet, societies grew smarter, so there must be herd intelligence” breaks at “so individuals must have gotten less intelligent”.
I think/guess that argument may have merit when replacing brain volume by number of neurons (https://en.wikipedia.org/wiki/List_of_animals_by_number_of_n...)
Einstein’s brain reportedly was below average size.
That’s a n = 1 example, but there also is a 50/50 example: man vs women. on average, the brains of males are about 10% larger than those of women (https://en.wikipedia.org/wiki/Neuroscience_of_sex_difference...). That doesn’t show up in intelligence differences (https://en.wikipedia.org/wiki/Sex_differences_in_intelligenc...)
Even only looking at males or females, I don’t think larger (fe)males tend to be more intelligent.