Why bootloader sizes differs from 0.5 to 8 kilo bytes for different boards? 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.
Bootloader of the Mini and Uno are pretty much the same. (I can't find the source-code but the hex files are only slightly different). 

The 2k is because they haven't updated mini328.upload.maximum_size in boards.txt. Probably because that would break the board for users that haven't updated the bootloader yet.

The 2560 is using a stk500v2 compatible boot loader instead of Optiboot (uno and mini), because optiboot only supports upto 64kb addresses. Not sure why it is that much bigger though.