Brian is correct, the SEP Agent definition sizes will NOT be decreasing in size, in fact they appear to be growing every so many weeks. Last we checked, for traditional AV alone, they're well over 400 MB on disk.
What RU5 is supposed to bring is a reduced size in the manner in which the SEPM stores and serves updates. So if right now like us you retain the last 30 revisions, your FG_CONTENT of the SEP DB is likely over 30 GB worth on disk as BLOB data. This is what's supposed to be reduced, but nowhere near the percentages being posted here from what I have seen so far.
So the theory is, you can then store more revisions using the same space you currently have, or save space by retaining the same amount you currently do, and so therefore more deltas can be offered, reducing the chance of full update packages needing to be served.
However this will not reduce that initial full update a SEP Agent needs, nor the overall storage requirements on endpoints. Those who are part of the RU5 beta program will have seen what I am talking about.
The only way the full definition files a SEP Agent requires can possibly reduce at a decent level is if Symantec make the hard decision to re-architect their entire engine and definitions design, and I do not see this happening any time soon.
Having worked for a once major AV company a decade ago, these kind of decisions take years of work, because you introduce great risk in re-designing something like this. It's not a "walk in the park", it takes a LOT of careful, design, implementation, and LOTS of drawn-out QA.