a quick place to start would be the systemd services that get automatically started when you boot your system. when i did this in the past, i would google each service that was running to determine if i needed it and remove the associated software if i decided that i didn't.

(since you're using debian): if it's a fresh install, it would make more sense to start with a minimal install first like the netinstall image and then pick and chose what you want to put on top of it.

if your issue is that the distro is too bloated: there are other minimalist distro's out there (some are based on debian) and they've already gone through the hassle of figuring out what the bare bones minimum is for fully functional distribution that can serve a viable daily driver.

The first thing I do to, if I need to get the size down, is swap out Gnome for one of the X11 Windows managers, usually XFCE.

I usually do this by starting from the minimal install and building up, as schizo already suggested.

That said, I guess I would be remiss if I didn't point out that Linux Mint is an easy way to get Debian's core with the XFCE window manager.

Looks like Mint starts at 3GB - 8GB, depending on options chosen?

Disclaimer: It's honestly been awhile since I really paid attention to my own Linux install size, as long as it's below 40GB.

You can start with dpigs. Then start marking packages automatically installed with apt-mark. aptitude may be a good frontend when removing a lot of packages, you can mark entire categories, like libraries, as automatically installed.

Pay attention to the package headers when removing packages. You don't want to remove essential packages.