The average bitrate should be about the same at vbr and cbr. So if you compare the total filesize you will get the same numbers. VBR allocates the bitrate and use more on some parts when it is needed but use less when less bitrate is needed to get an average quality on all parts. CBR use the same bitrate on all parts of the file.
You can check the bitrate with bitrate viewer (
http://www.tecoltd.com) then you can see the difference.
Ronny