So I've read the forums and searched on the net for the last few days and found one user which was able to compress his apps to save space just like SL does and saved some GB and was wondering how this could be done. Thanks for your help
News to me that SL can compress apps and save space. I originally thought it saved space but it turned out that it simply did not install everything that Leopard did by default and calculated free space differently.
The latest version of XSLIMMER works on SL and does a great job significantly reducing the size (and improving startup times) of most applications.