Faster processor (1.5 GHz vs 1.42 GHz) & higher system bus speed (167 MHz vs 142 MHz) & a faster 64 MB VRAM video card vs a 32 MB one in the iBook are all contributing factors. Both used the same PC2700 333 MHz RAM, the iBook could have 1.5 GB vs the 12-in PowerBook's 1.25 GB.
Geekbench 2 figures put the PowerBook slightly ahead at 725 vs 713.

streakmachine.10centuries.org.