My 500gb ssd  has approximately 150gb free space. Prior to March 1, full backups using medium compression level have file sizes about150gb. Since March 1st, file sizes are now about 250gb. I'm using the same backup definition files to create the backups.

If you go to the Restore tab in Reflect, you should see your existing backups.  If not, click the "Folders to search" link and add the folder containing your backups.  If you click on any given backup, the partition map contained within that backup will be shown at the top of the Restore area.  That map will show all partitions included in the backup, their size, and how much space was in use on each partition at the time of the backup.  Compare amount of space in use between your Full backups.  If you don't have older backups anymore but have the logs for those backups, then go to the Log tab and check the logs from those older Fulls.  They too will specify how much space was in use on each partition that was backed up.

If you find a significant and unexpected jump in the amount of space used on your partition(s), TreeSize Free is a tool that I find very useful for identifying where space is being consumed.  You can have it map a folder or an entire drive, and either way it will sort child folders by size, and then as you drill into each top-level folder, the subfolders and files will be sorted by size as well.  This makes it very clear very quickly where your storage is being consumed.  Make sure you run it elevated (as administrator) if you want to map your C partition, otherwise it won't be able to map certain portions of the drive accurately.