You can shrink every Debian/Ubuntu installation to a minimum. An Ubuntu installation does even take less harddisk space than the same installation under Arch Linux.
Why? Because ArchLinux always installs all development headers and GCC for a package. Under Debian/Ubuntu you have the choice. You don't need GCC? Than deinstall it? Software-development? Bah! I don't need C-header-files, kick it out.
Some weeks ago, i customized an Ubuntu installation, so it does only need 128MB RAM and less than 500MB HD-space. All that was installed was the basic system (via Alternate-CD), IceWM and Midori as a browser. No CUPS, no HAL, no whatever.
So don't tell me only Arch allows you to install whatever you want.
Arch does you only make feel you have more control over the system, because you have to start from scratch. "Normal" distributions are full-fledged, after installation. So first you don't have a choice but you can customize the installation afterwards.
That is the only difference between Ubuntu and Arch.