May be Inkscape has accurate implementation but Inkscape document
model is  highly non effective. There is a simple comparing test. Just create
1000 rectangles, convert they into paths and measure memory usage in
Inkscape, CorelDRAW and sK1. You will get approximately following numbers:
   
                                Cold start/With document
Inkscape 0.48             62Mb/140Mb
CorelDRAW X3           47Mb/63Mb
sK1 0.9.1                      51Mb/62Mb

So for the same drawing applications use:
Inkscape 0.48             78Mb
CorelDRAW X3           16Mb
sK1 0.9.1                    11Mb

Cairo branch is at 78Mb/85Mb using that reference file (Ubuntu Natty, 64-bit). That puts the additional usage at only 7Mb.

Cheers,
Josh