@marco van de Voort
I don't think it's just that. I think they've spent however many years adding features, and are now taking a moment to consolidate. Microsoft are doing the exact same thing. The rumoured complete rewrite of fuggly Quicktime should also help; my interpretation of it now being "fully hardware accelerated" is that it'll be one of the first things to use OpenCL and offload large chunks of processing onto the GPU. Probably raw OpenCL code is larger than raw Intel or PowerPC code, but bits like having the multi-resource scheduling (ie, a task scheduler that can manage multiple cores and GPUs) at the OS level probably dramatically cut the amount of self management that goes into other frameworks?
Apple had been obsessed for years with separating out multimedia stuff so that it can be processed by whatever black art the OS wants so as to be able to utilise Altivec and make up for the otherwise podgy progression of PowerPC processors, I guess now there's a more unified system, a bunch of things will naturally shrink.
Oh, and then the other 5.5gb is probably saved by not installing all the printer drivers or something.