When you upgrade from one Mac to another, you can use "Migration Assistant" to get all your apps, docs, and settings on the new Mac. This _also_ brings across all the leftover cruft from apps you no longer use, but whose data is still in your ~/Library folder (hidden, contains the working data for most apps)
Repeat a couple of times, and your current Mac will have large amounts of files that are leftover from your current mac, your previous Mac, _and_ even from your earlier Mac!
"Clean My Mac" will help remove this accumulated cruft.
Source: Mac developer whose product is guilty of using ~/Library to store a large database.