According to official web info bootloader sizes are 0.5, 2 and 8 kilo bytes for UNO, Pro mini and ATMega2560. I believe all three boot loaders do similar job of receiving serial link data and write to Flash memory.
UNO and Pro mini use same/similar ATMega168/328 chips but boot loader size are 0.5 and 2 KB.
ATMega2560 apparent has a more advanced MCU which presumably should use similar or even less memory to do the same task, but the size is significantly large at 8 KB.
Why sizes differ? Small difference may be explained by different development teams, but, should be that much different, from 0.5 to 8KB.