Why bootloader sizes differs from 0.5 to 8 kilo bytes for different boards?
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.