As far as I understand, chips are basically made of parts. CPU, of course, then one part for USB peripheral, another for USART, another for ADC and so on. Every peripheral is more or less a complete and isolated device. So if it's possible to get hands on this peripheral datasheet (I think it's called IP) written for chip designers, that would reveal complete complexity of the given part of the MCU.

Also I don't really have an idea, but I wouldn't be surprised, if a very significant part of transistor budget has been spent on SRAM or something similar, which is not very fascinating.