Debian stable will always prioritize stability and provide you older versions of applications. Even Debian Sid(their testing/rolling release version) gives you less than bleeding edge versions of apps. You can always install your own versions by downloading from provider or building yourself but if you’re wanting more current software, I’d consider another flavor of linux.
You can always install other themes, icons, etc. to get the look you want, Debian is just the underpinnings of the desktop. Using XFCE there is no different than using it in another distro.
The size difference is because of preinstalled applications, as you suspected. Some call it bloat, others just understand that Ubuntu is trying to cater to “set it and forget it” user.