Page size variations in cups

[Pigeon <jah.pigeon@ukonline.co.uk>]

Printing on different printers/drivers, all nominally using A4, I get
peculiar differences in the page size used/reported on the test page.
Performance varies wildly too.

Printer  : Epson EPL5200 (HPLJ compat.)    
Driver   : HP LaserJet Series CUPS 1.1
Page size: 7.76 x 10.69 in
Speed    : Fast

Printer  : Canon LBP-4
Driver   : Canon LBP-4U, Foomatic + lbp8
Page size: 7.78 x 11.28 in
Speed    : Very, very slow

Printer  : Canon LBP-4
Driver   : Canon LBP-4U, Foomatic + cjet
Page size: 7.93 x 11.36 in (Too big: loses box round the edge)
Speed    : Very slow

The Lower-Left and Upper-Right values show similar variation.

I have spent the past 3 hours or so poking through the cups
directories and have failed to find anything to indicate why the paper
size should vary so. On the contrary, everything seems to think that
A4 is the same size - as it should. Has anyone else come across this,
and managed to figure out what file to edit to get everything to come
out the same size?

Also, any ideas why the two Canon drivers should be about an order of
magnitude slower than the HP one? The Canon processes only take maybe
2% CPU at most (Celeron 600), and the printer port hardware is the
same in both cases.

Pigeon