I haven't entirely read the following article, but it looks interesting:
http://msdn.microsoft.com/en-us/library/dd894051%28v=sql.100%29.aspx

Ann pointed out in her email some things like fragmenation etc ...

Without getting into a deep discussion into Firebird internals (which I
don't have the expertise anyway), I basically argue that compression
could help to reduce disk I/O and network bandwidth at the cost of extra
CPU cycles. For compression on disk, at the server and for compression
over the network on both, server and client.

Oldish, but an interesting read as well:
http://epic.hpi.uni-potsdam.de/pub/Home/TrendsAndConceptsII2010/compression_and_db_performance.pdf



Regards,
Thomas