Based on your picture, it seems that the two images are the same size.
But your G4 powerbook (unless it was the very last generation from last November) has a lower resolution. I'm guessing that the image is optimized for the lower resolution, which explains why it looks crisper. In order to make the image the same size on a higher resolution screen, you need to use more pixels, so the image needs to be "expanded" or stretched. If the images were both displayed using the same number of pixels, then it would show up smaller on the higher resolution (MBP) screen. Again, your picture shows them the same size, and I'm assuming you didn't stretch or shrink the images.
PowerBook G4 1.67GHz / MBP 17" C2D (soon...) Mac OS X (10.4.8)