We asked camera companies why their RAW formats are all different and confusing

No it doesn't. A DNG is just a container that can hold anything, including compressed data, just like the RAF.

Coincidentally, most proprietary RAW formats are just bastardized TIFFs, and DNG is also a TIFF derivative...

There is zero technical reason not to use DNG. Leica and Pentax use it just fine.

Please explain how can I convert compressed RAF to DNG with same size.

It's a very common mistake in free software to not design a system end-to-end. Free software people will design some kind of generic container format and go "look! you can put anything in the container!", declare the job done, and then not write tools which actually do put anything in the container, or tools which can make use of anything in the container other than one specific thing.

(See: ActivityPub)

DNG has nothing to do with free software, it's an Adobe format similar to PDF. It is an open standard, but it is covered by patents and it comes with a no-cost patent license (except that it's less open than PDF because it doesn't define the interpretation for Adobe-specific tags (which ACR/Lightroom uses), not that that matters for raw file in any way).

Of course that even Adobe DNG converter can do what the GP asked for (I just tried it[1]), not that I would recommend it for Fuji files. And not that it matters anyway, since the whole point is producing DNG files directly, not converting them.

Edit: on my Fuji X-T5 files, using mosaiced data with lossless JPEG-XL compression (supported by MacOS 14+, iOS 17+, and the latest Lightroom/ACR):

    edengate:1$ ls -l
    total 163664
    -rwx------@ 1 aram  staff  40894672 Apr  4 15:25 DSCF1483.RAF
    -rw-r--r--@ 1 aram  staff  42894224 Apr  7 16:51 DSCF1483.dng

[1] https://llum.chat/?sl=3MCDl4