Don't even worry about such things. Your Mac is running perfectly, I promise. But I must admit, I'm a little curious as to why you'd want to downgrade to Tiger.
As for where your HDD space all went -- OS X is pretty big. A standard install of Leopard eats up about 12GB, including 3.7GB of printer drivers. Add iLife '08 and iWork '08 and it'll nearly double to 22GB, including almost 4GB of just Garageband and iDVD support files. So yeah, that's how a 74GB drive can suddenly have only 51GB free and yet it feels like you haven't put anything onto it. Tiger is probably within 10% of this size.
If you're curious about exploring Mac OS X file structure, and relative file sizes, I'd recommend downloading Whatsize from Macupdate.com.