How to install source code for RHEL 8.5 and build modules ( like nouveau )

Hello
I have been trying to install RHEL 8.5 source code ( for kernel + modules ) in order to rebuild some of the drivers ( gpu/drm/nouveau ).
I have downloaded the rhel-8.5-source-dvd.iso for RHEL 8.5 and extracted the file kernel-4.18.0-348.el8.src.rpm.
From kernel-4.18.0-348.el8.src.rpm I extracted the file linux-4.18.0-348.el8.tar.xz and finally I unpacked linux-4.18.0-348.el8.tar.xz.
I was now able to buid kernel and modules, and only module right now, that I want to load is 'drivers/gpu/drm/nouveau/nouveau.ko'.
So I 'signed' drivers/gpu/drm/nouveau/nouveau.ko by using scripts\sign-file after I stripped the file for symbols.
Finally is used 'xz' to compress it, so my final modules looked like this:
-rw-r--r--. 1 root root 418216 Mar 18 11:45 nouveau.ko.xz

Original filesize for nouveau.ko.xz is:
-rw-r--r--. 1 root root 648036 Dec 9 04:24 nouveau.ko.xz

I have attached 'modinfo' for the 2 modules ( orig + new )
Why is the difference in size so big ??:
orig: 648036
new: 418216

Best regards

Ole