My raster_brushes.propcol file (in the ~/Library/Containers/com.seriflabs.affinityphoto/Data/Library/Application Support/user/ folder on my Mac) was a hefty 1.37 GB & filled with brush categories I never use, so I decided to reset it to defaults using the Preferences > Miscellaneous > Reset Brushes option.
To my great surprise, the file size did not change at all, even though there were only around 1/6 as many brushes in the default categories. Next, I tried using the CTRL key startup option & selecting only the "Reset Brushes" option, which (to my relief) reduced the propcol file size to a mere 11 KB.
All the default brushes seem to work as expected after either reset & the propcol file size has remained 11 KB through several restarts of Affinity Photo (version 1.7.2).
So I would like to know if this is the expected behavior, caused by something weird specific to my Mac, considered to be a bug introduced in a 1.7 update, or what.