I just went digging for why Ethernet jumbo frame size is typically 9000, and found out that the reasoning is "until equipment supports much more than 9000 byte frames, we can use 9000, because it is an easy number to remember" https://docs.globalnoc.iu.edu/i2network//jumbo-frames/rrsum-almes-mtu.html
@whitequark Some more historical perspective: one of the biggest early beneficiaries of big frames was NFS. NFS at that time had standardized on a block size of 8192. 9000 was a round number a little bigger than 8192 + UDP + IP + Ether headers.