Fix your settings in WinRAR. Even on WAV files, when I use the best possible settings in WinRAR, the difference between a 7z file and a RAR file is a few megabytes. On a (once compressed) 300mb RAR file, the same files (completely the same, no differences at all) in a 7z were 297-298mb. So, you just need to get WinRAR to be more efficient. Also, in most cases that I've seen, WinRAR will compress (and often extract) files faster than 7zip does.
To optimize settings in WinRAR, make sure you're using "Best" for compression, and that you enable "solid archiving." Solid archiving will make it so that any data that is shared among files will be counted only once. ZIP doesn't do this, but 7zip does (by default) which is probably why you're seeing big differences between RAR and 7z. Hope that helps. Mouser X over and out.