30GB+ used on freash Arch install+KDE? Why?

You can try using # du -h -d 1 / to locate the largest directory under /. Once you've located the largest directory, replace / with that directory. Repeat that until you find the culprit (if there is a single large directory).

EDIT (2024-07-22T19:34Z): As suggested by @DarkThoughts@fedia.io, you can also use a program like Filelight, which provides a visual and more comprehensive breakdown of the sizes of directories.

You can use Filelight which is much simpler and more visual.
- But it doesn't make you feel like hackerman
- Agreed.
- ncdu for the terminal. Also enables you to delete folders/files.
You're a life saver I finally found the culprit
- Do tell! We need a follow up :)
Df does that too, or did you mean du?
- Whoops! You are correct — I have updated the original comment. I'm not sure why I wrote df instead of du. This is a good example of why one should always be wary of blindly copying commands 😜 It begins to teeter on being potentially disastrous if I had instead wrote dd.
Or you could use baobab to do the same thing if you want an answer within 10 minutes.
- Or dust if you want it fastest with a pretty graph

Maybe you have a swap file that happens to be 16GB ?

I only allowed 4G for swap, maybe arch enabled zram and it used 8GB by default and I actually don't need to create a swap partition?
- Arch doesn't really do anything you don't tell it to do during installation.
  That's the entire point. After installing Arch, you know what your system does, cause you configured it.

It might have something to do with the dolphin you're keeping in there.

?
- Dolphins are quite large

Try the following command to list all installed packages sorted by size [source]:

LC_ALL=C.UTF-8 pacman -Qi | awk '/^Name/{name=$3} /^Installed Size/{print $4$5, name}' | LC_ALL=C.UTF-8 sort -h

There may be some unexpectedly large packages installed.

Keep in mind that a part of the filesystem will be reserved on creation. Here if I create a completely empty ext4 filesystem with:

    
truncate -s 230G /tmp/img
mkfs.ext4 /tmp/img
mount /tmp/img /mnt

Dolphin reports "213.8 GiB free of 225.3 GiB (5% used)"

Try running pacman -Scc to get rid of the pacman cache.

Also: How did you install KDE?

I used "sudo pacman -S plasma sddm"

    
ncdu /

sounds ok for me if you install the full KDE Plasma + all applications package group and add some basic software like LibreOffice

I used minimal plasma (pacman -S plasma)
- plasma-desktop is the minimal one.
- use plasma-desktop, that's the actual minimal one.

Freash on a leak