Data File Size Stays Same Whereas Log File Size Is Increasing
Probably because the function FILEPROPERTY reports either whole pages user or even whole extents used (documentation is unclear on which, but that detail doesn't really matter). The point is that storage inside the database file is pre-allocated for data. That means that some storage is pre-allocated and now you have free space that can accommodate some new rows until it has to grow the next time. This pre-allocation inside the database files doesn't really happen in a transaction log file - at least not the level that FILEPROPERTY reports on.