There is a uservoice already for this and it's been a pretty big topic for awhile now. Many people do not like the look of Teams and they just said they are going to work on a compact version for people that choose to set it that way. ETA is probably a ways out. As Linus said, zoom in and out, (Ctrl + mouse wheel), Profile pic > Zoom section etc. can help but there still is a lot of wasted space in Teams.
I'm not sure if it's the same for everyone but every time I Exit out of Teams and restart, the Theme I picked (Dark), is reset to the Default and I have to choose it again. The same behavior throughout all my devices.
I tested to change to dark theme on my computer, quit teams and then start it again and the dark theme was still there. Try to reinstall Teams, uninstall it first and the remove all files from %localappdata%\Microsoft\Teams %appdata%\Microsoft\Teams, after that install Teams client again.