Manual:$wgCompressRevisions

Details
If this is set on, all future revisions will be compressed on page save, but only if zlib support is available and enabled in PHP.

We can also compress text in the old revisions; for this, use the script located in directory /maintenance/storage. Any compressed revisions will be decompressed on load regardless of this setting but will not be readable at all if zlib support is not available.

Note
Once you enable, the compressed texts will no longer be searchable/replaceable via SQL scripts. Extensions which require such scripts may not work properly.

Using the script periodically instead of enabling  may avoid this problem.