For a directory with about 30 files in it (all scripts, in fact), I compressed three files. The tar is a tar, the format is never compressed. The zip and 7z were created with compression turned off. Results:

-rw-r--r-- 1 me me 501760 2007-12-14 19:38 test.tar
-rw-r--r-- 1 me me 472411 2007-12-14 19:38 test.zip
-rw------- 1 me me 469089 2007-12-14 19:51 test.7z

Now I will bzip2 the tar and zip files, and then additionally create a zip and 7z file. For all commands I specified compression level 9 (best). Results:

-rw-r--r-- 1 me me 267195 2007-12-14 19:38 test.tar.bz2
-rw-r--r-- 1 me me 267668 2007-12-14 19:38 test.zip.bz2
-rw-r--r-- 1 me me 266118 2007-12-14 19:53 test.zip
-rw------- 1 me me 233809 2007-12-14 19:51 test.7z

Interesting things to note:

• Even zip by itself is smaller than the zip.bz2.
• The tar starts out bigger than the uncompressed zip, but after bzipping the two, the tar.bz2 is clearly smaller than the zip.bz2, although the ordinary .zip is smaller still.
• The 7z is a clear winner in the compressed file category.
• The 7z is even the winner in the UNCOMPRESSED file category!

Incidentally I tried creating a solid archive as well (in this situation 7z groups the files together and then compresses the whole set in blocks) as I expected this to be smaller. But it turned out to be exactly the same size, so my input files mustn't be small enough to pay off for that optimisation.