Since mozilla still does not install to where I point the --prefix I still just tar everything in mozilla/dist and extract it in /usr/mozilla. Maybe you could write a paper about how to get a clean install. My experiences are that some earlier versions got me some gig's of binaries after compiling. Then, 0.9.7 actually was "only" 245 Mb "small". The following 0.9.x releases were about 1 gig size again. RC1 was still more than 700 Mb big. Then RC2 was only 183 Mb "small". This is the smallest size ever. Now RC3 is 863Mb big. I don't understand these drastic changes, they must be related to some more or less softlinks from inside mozilla/dist or whatever. but, I marked this problem as crucial because if 1.0Final is still that big for me then I will not use it.
The difference in size is because you are building the debug version, which is the default. Click on the Build Configurator link at http://www.mozilla.org/build/unix.html to get the list of current build options. The one that you'd be most interested in is --disable-debug. The 'make install' bug (bug 56601) has been fixed on the trunk for awhile and was just checked into the 1.0 branch earlier this afternoon.