My ACDsee catalogue database has been left unoptimised for the last few years. Its managed to grow to the size of a monstrous 1.6gb.
You can access it by selecting "Database" and then "Optimise Database ..." in the main menu. I run the optimisation with both tables/files and remove orphan options selected.
After running the optimisation, the database has been reduced by half to 860mb.
There are more uncommon space saving tips here.