Unpacking-repacking the stock, unmodified, system.img (Nougat) leads to unbootable device.

Hello everyone!

I ask for your help regarding unpacking-repacking system.img of an Android 7.0 Nougat.
In order to start a porting process, I have just tested several system.img tools (for unpacking-repacking it).
I have tried with the stock ROM (for testing purposes). Just unpacking-repacking the original ROM. But the system is unbootable, after flashing the new image.

What I did:
- From boot.img/ramdisk/fstab I have deleted the "verify" tag from /system partition ---> which should have disabled the dm-verity
- I have unlocked the bootloader (via fastboot oem unlock). It is confirmed at Logo stage.
- From boot.img/ramdisk/default.prop I have set the ro.secure to 0 (from 1), ro.adb.secure to 0 (from 1)

What tools I have tried:
- All Windows tools found on the Internet for packing-repacking system.img
- Under Linux, simg2img and img2simg, etc.etc.

Results:
- EVERY time, the system.img file resulted is smaller (with several MB!!) than the original one! Without any modds!
- System unbootable. (after flashing via SPFT)

The problem is 99% with the image tool, because the output is smaller in size than the stock ROM.

They are not relevant, but I provide some information about the device.
[INFO]
- MT6753 chipset
- Device: VOYO Q101, 2GB/32GB
- Android Nougat 7.0, kernel 3.18.35+

PS. Unpacking-repacking a 5.1 Lollipop is okay. The repacked image has the SAME size as the original one. Unfortunately, something happens with Nougat images and ALL extracting tools.

Any ideas? Thanks!