Your guess is correct. During the Canopus digitizing your SVHS input is converted to 720x480 NTSC. The digitizing produces a complete video input that PRE captures. Averages of around 13GB per hour are typical. So from the figures you have quoted your digitizer has done its job properly.
Digitizing tapes was the subject of a long topic discussion earlier this month - you might find it interesting: http://forums.adobe.com/message/5906986?tstart=0. While that discussion was relating to VHS tapes, SVHS still only produces around 400 usable horizontal lines, so the Canopus will have captured it all at 720x480.
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Thank you Neale.
Your captured avi is compressed, it uses the DV codec. If it were uncompressed the file would have been 5x larger.