How does the size of the filter, and type of media affect this flow rate?
The size of the filter affects flow rate as presented in the following equation: The volume of MEDIA (cubic feet) in the filter unit divided by the flow rate (cubic feet per minute), is known as the Empty Bed Contact Time (minutes). To effectively remove chlorine (aka: hypochlorite), the contact time needs to be at least 2/3 minute. To effectively remove any of the chloramine compounds, the contact time needs to be at least 6 minutes. For the typical 10-inch carbon filter unit, that equates to needing the 1 and 0.1 gpm flow rates that I mentioned above. For the big carbon tank that you mention, that flow rate can be assessed by reconfiguring the formula above. Its obviously a much higher flow rate.
Loose or granular activated carbon (GAC) is less dense than the more modern carbon blocks that are now available to consumers for the 10- and 20-inch filter canisters. Therefore, carbon blocks are now preferred over GAC filters.