ThumbnailTools::GenerateThumbnailForObjectToSaveToDisk seems to not return any of the memory it uses for thumbail generation until the editor itself is closed?
After running it on about a thousand textures, my memory usage ballooned to 21 gigabytes of memory used for about 800 megabytes of actual data on disk (including the thumbnails). Same thing with about 500 mb of meshes, which turned into 12 gb of memory in use by UE after thumbnails were generated for them.