For the best jpeg quality first export the plan as .pdf and then convert to jpeg. you can get the best output.
Otherwise, the jpg format has been over optimized for file size. For example a test project saved as a .bmp is 5,963 Kb compared to 50 Kb as a .jpg. We are looking to give the user the option to select file size/image quality. Until then you can save as .bmp, load into Microsoft Paint and save as .jpg (where file is 50 Kb but better quality).