just upgraded from 11.10 and been using Ubuntu since 9.04. The Canon driver in 11.10 worked OK with A4 paper, but it looks to me as if it prints to A5 size when you select A4 in Ubuntu 12.04.
To amplify, selecting A4 paper and 'fit to page' gives a print preview which is correctly sized and as expected, but when it commits to paper, the print area is half size.