Very impressive, i've done some similar things in (ArcGIS / ArcScene) and (QGIS / qgis2threejs)
Geo-referenced mines plans combined with Google Earth images on Lidar.
But have always had issues with the final output files being either huge or not in a format Sketchfab likes.
Your file sizes seem sensible, can you give me any pointers on the software/workflow you used ?