EV will increase Outlook OST size ???

I think you can clearly see from the links that I've given that this isn't something 'unique' to the way that Enterprise Vault is involved.  Particularly this from MSDN:

<snip>

When deploying Cached Exchange Mode for Outlook, be aware that users' OST files can increase in size by 50 percent to 80 percent over the size of the mailbox reported in Exchange Server. The format Outlook uses to store data locally for Cached Exchange Mode is less efficient than the server data file format, resulting in more disk space used when mailboxes are downloaded to provide a local copy for Cached Exchange Mode.

</snip>

The reason that I asked about whether or not Vault Cache is involved is because of the testing ...  I imagine that when you take a non-EV user, with a 500 Mb mailbox, what you're doing is opening Outlook and letting it build a new OST file.  Once built, you're closing Outlook and looking at the size of the OST file?

In other words you're not doing any activity on the OST file.

When Vault Cache is enabled it will trawl through the OST file.  This will cause the OST file to grow... in part because the indexes which Outlook maintains for the OST file will be built and amended.

I'm not really sure what problem you're trying to discover, or fix here..  it's the nature of the beast unfortunately.

If you took that 500 Mb mailbox, on a non-EV enabled user, and let that user use that OST for a period of a few weeks, you'll see that the OST file is going to be 'much' larger than the initial build size.

Hope that helps,