Execution of test runs, either manuel or automatic, will create a bunch of diagnostic test data that can cause the the databse to crow quite rapid.
For vNext build /test jobs you define a retention policy, that will tell TFS how many days you want to keep the data (default 90 days). This job will run automatically every night.
But when using xaml based builds the retention policy is triggered on each build, so if you stop using a build definition, e.g. when you have relased and clone the definition, old builds and test data will be kept forever, until you trigger a build.