It's not so much the 'core' as the memory. For example it looks like the RTL8367 has 16KB of program memory, 32KB of data memory, 256bytes of SRAM and a 12KB ni

It's not so much the 'core' as the memory. For example it looks like the RTL8367 has 16KB of program memory, 32KB of data memory, 256bytes of SRAM and a 12KB nic buffer. These memory cells will make up the bulk of the transistors in the 8051 part of the chip and tend to be points of manufacturing failure. In many cases the chip would be designed with more memory than it needs under the assumption there will always be a few failures. So if you need 16KB, you design in 18KB and turn off the rows/banks that fail or test as the worst. If too many fail you sell it as next memory size down, or in this case you zap a couple fuses and sell it as the part without an MCU on board. Of course this doesn't always work, sometimes you need more of the non-perfect parts than you have to fill orders so you just zap the fuses anyway.