Default [Q] Need some help with modifing kernel


I'm not sure if this is just a dumb noob question or not,
but if i unpack my boot.img file from my moto g, using android kitchen (cygwin) and if i repack it again, without modifying anything, the size of my kernel goes from 10MB to 6MB, and the created boot.img can't be used anymore.

Does anybody have an idea what i do wrong? I need this to port a kernel (replace bootclasspath)

Thank you in advance!