Head on over to
www.cheapbytes.com to order up some cheap-Unix-happiness... BSD and Linux, at a few bucks a pop.
Although the best way might be a how-to book... It gives you a Unix, then teaches you how to install, configure, and use it!
The reason so many Unix's are bloated are they they are designed to work on very generic hardware, as well as specialized. People want to make sure all of their hardware works properly with their operating system. Also, most Unices come with hundreds (sometimes thousands!) of bundled / free programs, utilities, applications, desktop themes (entirely different desktop managers too), etc, etc, etc... There are some "mini" distributions out there... some are only a few floppies in size...