Is there some specific reason you want to change the display other than you do not like the large numbers? AFAIK Amanda has always used KB for these displays and it seems to work.
If there is some good reason, grab the source code and make it work like you want and send the patch back. That is the joy of open source.
You might check the zmanda forums for more specific answers. You will find them at
http://forums.zmanda.com . There are specific forums on Amanda as well as Zmanda.