Yeah I'm on a 10.5. Why would it be slower depending on window size?

Because Chrome has a multiprocess rendering architecture, and OS 10.5 doesn't provide a good path for hardware-accelerated rendering across processes. This means Chrome on 10.5 has to render each frame on the GPU, read it back to main memory as an image and then repaint it to the screen elsewhere. The bigger the window, the bigger the image and the longer this readback/copy takes.

10.6