It's simple Karsten, well sort of!
The actual application is 2.08GB but when you have installed it you will find that Software Update will allow you to download 1.2GB of extra content, which will probably expand in your computer to double that amount.
This extra content may be stored in HD>Library>Application Support>Final Cut Studio>Motion.
This folder (Motion) has about 8GB of content from FCS 3.
When I downloaded the SU I quickly whipped the downloaded file out of HD>Library>Updates to stop it from automatically being installed and the .dmg trashed.
I then installed it the usual manual way and it told me that some 700MB would be installed.
I don't know whether this material went into the original Motion contents folder or whether it is stored somewhere else.
I hope that clears it up for you!