Linux

This is a real can of worms, as you can see from what other people have said. Let me chime in with two things: first, it depends on the partition and filesystem type, which usually depend on which distribution you're using. For example, Debian, Ubuntu, and Linux Mint default to the ext4 filesystem, which is easy to grow and shrink. Most Fedora derivatives use btrfs, but Fedora Server uses xfs, which cannot be shrunk. So you'll need to know what filesystem your system is running in order to proceed. The safest thing to do if you go this route is probably to backup everything and reinstall the OS, picking the new size from the installer. However, depending on the filesystem, resizing may be possible. Regardless, this is potentially dangerous and the first thing you do should definitely be to back. up. everything.

And second, if you've got a big filesystem that's mostly empty, the easiest way to solve this may be to simply copy whatever's filling up your smaller filesystem to the bigger one, rename the original, then symlink the location on the bigger filesystem to its original location on the smaller one. Then test to make sure whatever you changed didn't break anything. If everything worked, you can delete the renamed original. So for example, from the terminal, I'd do:

# Assume the smaller filesystem contains /home and the larger is at /mnt/bigdisk
$ cp -a /home/USER/.local/share/Steam /mnt/bigdisk
$ mv /home/USER/.local/share/Steam /home/USER/.local/share/Steam.orig
$ ln -s /mnt/bigdisk/Steam /home/USER/.local/share/Steam

Even though this is probably what I would do, a word of caution: don't do this if the stuff in question is required for the system to boot. And if you don't know, don't try it (or let it be a Learning Experience)! Generally, things under your home directory are safe, so if Steam's installed there, that should be fine. Good luck!