It also depends on the distro and how the packages are structured. I don't know how Nix does it but Debian for example likes to split packages into the bare minimum installable, and you get development headers and debug symbols as separate packages. So every library can be 3 packages, whereas on Arch you usually get the whole library as a single package. So out of the box, Debian systems will have many more packages. Nix could be somewhat similar. On Debian, especially if you install Python/Ruby/NodeJS apps (or even all 3), it'll also pull every single dependency as an extra package as well. Everything that'd be node_modules will pull a nodejs-xxx package. That's hundreds of packages right there.

But also, some users seem to really like super slimmed down systems. If you install KDE or Gnome, or worse, KDE apps on Gnome or Gnome apps on KDE, you'll end up with a lot of packages when compared to someone using i3/Sway and mostly terminal apps and a few lightweight GUI apps.

Then on Arch for example if you install steam-native-runtime, you get like a hundred extra packages to provide the whole Steam runtime environment as native packages.

You'd have to compare package lists to see what differs. My Arch system has like 25GB worth of packages installed and it's like eh whatever. I got like an entire Python/Ruby/NodeJS/Java/Android/C/C++/C#/Rust dev environment plus a bunch of Wine/Proton stuff and Steam and Lutris. So my system is "bloated" by every standard but I do have a lot of stuff handy. I can just open up an Android project from GitHub and compile it right away because I have all the devtools already. Could I slim it down? Yes. Do I feel like I need to? Nah, worst case it's using a good chunk of disk space which is nothing given I have an NVMe, 2 SSDs and 2 HDDs. 25GB with 6TB of usable space is 🤷. This is my main workstation and entertainment station, I want those things handy whenever I need them.