This information is not about the memory in the application, but the occupied space from the allocated data storage for each unit. Basically this is the place occupied by messages received from the device.
When the amount of stored data reaches its maximum size, then after a while the oldest data will be deleted automatically, and the new ones will be recorded. The application continues to run.
However, the more messages arrive and the more the storage is full, the shorter the retention period of the oldest data. You can see the approximate storage period of data at the moment in the unit menu - Settings - Common - information is located under the information about the occupied space in the storage.
Optionally, you can increase your storage space in the premium version from 50 MB in the free version to 100 MB in the paid version.
You cannot selectively delete data, the oldest are deleted automatically when the storage period expires and the maximum data volume is reached. You can only, if you wish, delete all data permanently by deleting the unit itself.