There's also a difference between lossy and lossless compression...and it's possible the capture software could be doing either, or neither.
I use the software that came with the camera (EZPlanetary) and it does the job, but if it's hard to decipher from what you're using it might be worth trying something like Sharpcap or Firecapture. In EZPlanetary I can set the exposure time (which determines the frame rate), resolution, gain and colour balance. There may be more but I haven't played with them
Obviously, for this sort of experiment it doesn't need to be plugged in to the scope, as a raw capture from the sensor should work out the same size (for a given resolution and number of frames) regardless of what it captures.
Also, do you have the option to select a Region of Interest? Or do you use the lower (and faster) resolutions? What you mentioned before about 10 or 5fps sounds suspiciously like the original Neximage and I'd be really surprised if that held true for the Neximage 5 which is a whole kettle of fish different from the former.