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

Compressed RAF (Fujifilm, 24MP) ~20 MB

DNG (24MP) ~90 MB

It cost about 4 times more to store RAW files in DNG format.

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

To test this, I downloaded a random RAF image from this gallery https://mirrorlesscomparison.com/galleries/fuji-xt2-sample-i...

    $ /Applications/Adobe\ DNG\ Converter.app/Contents/MacOS/Adobe\ DNG\ Converter DSCF6001.RAF
    SPL-LOG-1002: starting logger thread
    *** GPU Warning: GPU3 disabled via cr_config at init time. ***
    SPL-LOG-1003: terminating logger thread
    SPL ~DefaultMemoryManagerImpl bytesAllocated = 0
    $ ls -la DSC*
    -rw-r--r-- 1 danielh staff 50377216 2025-04-07T10:59:30 DSCF6001.RAF
    -rw-r--r-- 1 danielh staff 30747896 2025-04-07T11:00:13 DSCF6001.dng

Maybe your method of converting to DNG is embedding the original RAF image and ... something else?

He probably compares a mosaiced RAF with a debayered (linear) DNG. Just... don't do that. Use a mosaiced DNG. And certainly don't embed the RAF file. Plus with lossless JPEG-XL, the difference is trivial (under 5%):

On my own files:

    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

I downloaded a sample RAF file from the internet. It was 16 megapixels, and 34MB in size. I used Adobe DNG Converter, and it created a file that was 25MB in size. It was actually smaller.

Claiming that DNG takes up 4x space doesn't align with any of my own experiences, and it didn't happen on the RAF file that I just tested.

Lightroom cannot do that. I'm not sure if Iridient X-transformer have some options for this. I always ended up with massive files.

Also, I'm not confident to replace entire RAF collection with converted DNGs and delete originals.

Iridient X-transformer produces a linear (demosaiced) DNG, that's its sole reason to exist, to do a better job at debayering than other software like Adobe. So it wouldn't make any sense for Iridient X-transformer to produce a mosaiced DNG because that would be basically a noop, the program wouldn't do anything.

So yes, of course that files produced by Iridient X-transformer are large, they are linear files. They are exactly three times as large because there are three color channels, four times as large if you also embed the original.

There is zero reason to convert RAF files to DNG files if you camera produces RAF files. The discussion we're having here is cameras producing mosaiced DNG natively, which as I hoped I showed you wouldn't come with any size penalty. The DNG can use modern lossless compression techniques, and can encode the same mosaiced (not debayered) data. And it works in every program, unlike RAF which always needs to be reverse engineered for every new camera release.