Rate of Retrieve
Rate of retrieve, the major selling point of a large-arbor reel, is a function of the diameter of the reel spool. Let's define a standard-arbor reel as one with a spool arbor or hub approximately 1 inch in diameter. On such a reel, if the spool has only a few wraps of backing on it, one revolution of a (non-multiplier) reel will retrieve little more than 3 inches of line. (Remember your high school geometry?) Conversely, let's say a large-arbor's diameter has been increased to 2 inches. With this one change in the reel, you're able to crank in line at almost 6.5 inches per revolution with a similarly almost empty spool. Twice as much is 100 percent more line - on every crank!
During a fight, the rate of retrieve becomes a function of the diameter of the portion of the spool filled with backing and line. Since the effective radius of the spool increases or decreases as the line comes in or goes out, the rate of retrieve increases or decreases as well.
Increasing the arbor diameter only will do nothing to change the rate of retrieve with a full spool since the overall spool diameter hasn't increased. Instead, it will serve only to decrease the usable capacity of the reel. If, however, we increase the arbor of a reel and the overall spool diameter, we will increase the retrieval rate at both near-full and near-empty spool situations without losing capacity. Well-designed large-arbor reels, with both larger arbors and overall diameters, both increase the rate of line retrieval and retain sufficient backing capacity.
The change in effective diameter and rate of retrieve is more than a two-dimensional problem, however. Since fly lines and backing occupy space, we're really dealing with a three-dimensional issue. This third variable, the width of the spool, affects rate of retrieve as well.
Harking back to that geometry book, realize that a reel spool is simply a short cylinder turned on its edge, with a second cylinder, the hub or arbor, inside the first. Think of a doughnut. The hole is equivalent to the reel hub (arbor), and the edge of the doughnut is the outer edge of the spool. The actual edible part (let's call it "goody") is equivalent to the useable space on the reel spool, or volume. The amount of goody can vary by changing any of three dimensions. If we increase only the hole or arbor diameter, we have less goody. If we increase the outer diameter, we have a larger doughnut and thus more goody. Finally, if we increase the width, we have a fatter doughnut, and thus more goody. Obviously, the dimensions of each can be varied simultaneously to achieve a reel maker's specific requirements.