Repacking system.img issue

Hi,
i'm trying to customize the system.img for my own purpose and flash it via Odin.
i'm running Ubuntu 14.04 TLS.
i did some tests and non of them worked, the flash was succeeded but i get soft brick (bootloop)
so, what is did?
these steps, please correct me if i did something wrong.
1)simg2img on the extracted system.img from the stock tar.
2)mount ext4 type the converted system.raw using the following command: mount -t ext4 -o loop img.raw /mnt/

now i want to repack it without making changes (for tests), using the following commands:
1)make_ext4fs -s -l 3145728000 -a system system_nonbloat.img /mnt
2)sgs4ext4fs --bloat system_nonbloat.img system.img

the sizes are not equals. (~30mb less than the original system image).

someone know's what i'm doing wrong?

thanks!