Not an expert, but what I believe differencing disks do for you is
this:
- Base any number of new guests on one configured and working guest
- The base VHD is *never* written to so it can be set readonly
- *All* data you ever save in a derived guest go to the diff VHD disk
file and this includes the Windows swap file....

This means that the differencing disk will expand depending on how
your applications write data to it.

Now, the crucial question is to what extent the applications do change
the data and here I don't know. But a database server is likely to
change the contents of its files and in the case of MSSQLServer this
means the mdf and the ldf files (since these are normally the only two
files in a database irrespective of the number of tables). Other
database engines spread their tables into separate files. What Oracle
does I don't know.