lesdmd Loc: Middleton Wi via N.Y.C. & Cleveland

Usually I export from Lightroom 4 using a 16 bit Tiff format, for post processing, to either Topaz FxLab or Photoshop CS6, depending on what I need to do. These tiffs come back, from either through Topaz or CS6 the same size. Occasionally, probably because I started on computers when hard drive space was precious, I will export an image that I want to save but with which I am not totally in love, in jpeg format. When it comes back into Lightroom, after processing through FXLab, the file is literally 1/5th (1mb versus 5mb)the size to which it is saved, at highest quality, from Photoshop.
Does anyone know if the issue is with the way Lightroom is processing the conversion from DNG to Jpeg for export, or the way in which Topaz is saving the processed file? I want to save space, but not severely sacrifice the quality of the file.
Yes, I've posted the question to Topaz.