Photos disk space usage

I don't quite understand what Photos does: it uses 3 times as much disk space as Aperture for the same library.

My Aperture library with some 53k photos (~200GB on external SSD) and a few hundred videos (~100GB on external SSD) uses around 40GB disk space for thumbnails in various sizes, database and so on.

After a few hours of importing, the same library in Photos has grown to 119.9GB.

Settings are: do not copy images to library, no use of iCloud Photo Library (and thus no "optimize Mac space usage").

Inside the library file, the Thumbnails directory uses a sane-looking 16.66GB, Previews is 7.29GB, and Masters uses only 1.35GB, since almost all images are still stored in Aperture referenced masters on external SSD.

The largest directory inside the library is Resources > model resources, with a whopping 86GB for 147k objects (images).

I'm almost sure these are a result of my trying and then disabling iCloud Photo Library during import yesterday afternoon.

Since Photos does not delete those resources for some reason (disk space and RAM are always cheap and plentiful for Apple, I guess) – is there a way for me to get rid of it if I don't want iCloud Photo Library? Would Photos be smart enough to start culling images here when my system disk space, a paltry 256GB SSD, dwindles towards zero?