Also check the settings minor and major versions. Usually you need a few minor versions for accidental changes and a few Major for just proving who did (break) what.
It can add up mainly to your DB size, performance wise it should take a while before it impacts.
Note: when you change the number of history items to keep, it won't magically remove all old data. As soon as someone creates a new version of an object/page Kentico will clean up older versions that fall out of the limit for that specific item.