When exporting a spherical panorama, PS does not ask you to set your JPG compression level. It should just like it does when saving a regular JPG file. I noticed this when saving a file 8000px wide and my result was a 1.2MB file. PS applies the last setting used when a regular JPG file was saved, in my case 6, hence the tiny file size. WORK AROUND: Save any old JPG, set quality to desired level, then export your spherical panorama file.