Default [Q] Device can not boot up

My device is GT-i9500 and it have 11G storage space by flash a pit file.
So, If I want to flash others stock firmware, I should adjust size of system.img from 2760m to 2150m.
I did it, and flash it successful with stock boot.img.But question comes here: DEVICE CAN NOT BOOT UP, it just stop at samsung logo.
then I flashed a custom kernel named Perseus, it BOOT UP!!!
I do not know the reason why it can boot and stock not.
If anyone know, please tell me.
What the question is: If it possible, how to make boot.img to boot my device?

My english is not good, hope you guys can understand!

big THANKS!!