According to Wikipedia (I was able to verify the NFL ball size with official sources, but not the CFL size).
National Football League football is specified as; short circumference: 20 3/4 to 21 3/4 inches (527 to 540 mm), long circumference: 27 3/4 to 28 1/8 inches (705 to 714 mm).
Canadian Football League football is specified as; short circumference: 20 7/8 to 21 1/8 inches (530 to 537 mm), long circumference: 27 3/4 to 28 1/4 inches (705 to 718 mm).
The upper bound on the long circumference is bigger for the CFL, but the upper bound on the short circumference is actually smaller. The ranges overlap and the differences in the upper and lower bounds are small. Having bigger balls makes for funny slogans of questionable taste, but I'm wondering how much of a difference there really is.
I would guess that the manufacturers aim for the middle of the ranges, but even if the CFL used the biggest ball it could and the NFL used the smallest ball it could, the difference would be maybe a third to half of a centimetre in length and diameter. Admittedly, I haven't handled a lot of balls in my lifetime, but I'm surprised that anyone would notice the difference.
I read somewhere that there was a time when the CFL balls were noticeably bigger, but that was due to a manufacturing problem and the players didn't really like them. (Can't find the article back, though. Argh).
People keep repeating this like it's fact, but I'm skeptical.