72 dpi grayscale almost as big as 600 dpi colour

I scanned two pages of 8.5"x11" in 72 dpi grayscale, yielding a 450KB PDF file.

I then scanned a 11"x17" page in 600 dpi colour, yielding a 528KB PDF file.

The total area of these two scans are the same, so I was expecting orders of magnitude different in file size due to the pixel density difference and the use of grayscale versus colour.  Why is the KB savings so meager?

I am using a Xerox WorkCenter 7855.