From you question do you mean memory or the size of the file on disk? Also what is the issue you are trying to resolve? By knowing what you are trying to do will help you get a better answer from the group.
Note that the Assembly file itself will not be that large, in terms of the disk spaced needed, since virtually all of the geometry is actually being stored in the Part files of the individual Components. Granted, when an Assembly is opened, much of that data is loaded into memory, but even that can be controlled using certain approaches such as Partial Loading, Lightweight Models, etc.