boot.img changes after unpack-repack without edit
I have met some problem with boot.img and need some help if there's somebody know it.
I used unmkbootimg and mkbootimg, I unpacked a boot.img then got zimage and initramfs.cpio.gz, I repacked it without any edit but the new boot.img becames smaller, it changed from 13M to 7.9M, and the phone couldn't boot from new boot.img,
and there is no question if I use a boot.img extracted from an android3.x-version rom, but if the android version changes to 4.x, the unpack-repack process would lose much size...I tested on mi2s' rom......I don't know if there are some files that will be lost in unpack process, or say the process just ignore files other than zimage and initramfs...
does anybody knows why?