Reel manufacturers have chosen different dimensional approaches in the design of their large-arbor reels. Ted Juracsik designed his Tibor models from scratch to provide both the quicker retrieve of the large-arbor reel and sufficient capacity for line and backing when compared to standard-arbor models. The Orvis Vortex and Bauer MacKenzie series are similarly built from the ground up. Steve Abel took advantage of his considerable number of standard-arbor reels in designing his Super Series of large-arbor reels. The Super 12 (for 12- to 13-weight lines), for instance, begins with the Abel 4.5 (for 12- to 20-weight lines). By incorporating a large-arbor spool and extensive porting on both the spool and frame, the Super 12 will hold a WF-12 and over 300 yards of 30-pound backing (the standard-arbor 4.5 version will hold some 450 yards of 30-pound backing). The large-arbor version weighs in at only 10.4 ounces (compared to the standard 4.5's 14.1 ounces).
Other manufacturers have chosen to simply widen the spool on their large-arbor models, without increasing the overall spool diameter. This approach does provide some of the advantages of increasing the overall spool diameter, but not all, in my opinion. Finally, there are those few manufacturers who have simply increased the spool arbor. It is those reels that have generated much of the confusion and criticism of large-arbor reels, generally doing nothing more than reducing capacity without providing the other benefits of large-arbor reels.
Islander is one of several manufacturers that provide customers with plenty of information, listing not only the line capacity, spool diameter and width, but also the all-important volume. The numbers are interesting, and show the importance of evaluating any fly reel (not just large-arbors) in three dimensions. The LA 3.8 has a 3.8-inch spool diameter, a 1-inch spool width, and a volume of 5.94 cubic inches. The LA 4.0 has a larger 4.13-inch diameter, but a narrower 0.78-inch width, yielding a volume of 5.18 cubic inches. Finally, the LA 4.0W has the same 1-inch spool width as the LA 3.8, and the larger 4.13-inch diameter of the LA 4.0, giving a volume of 6.64 cubic inches.
The listed line capacities are WF-8F + 200 yards of 20-pound backing for the 3.8, WF-7F + 150 yards of 20 for the 4.0, and WF-10F + 200 yards of 20 for the 4.0W. Thus, while the LA 4.0 will retrieve line a bit quicker than the LA 3.8, its narrower spool has less capacity. The LA 4.0W, however, provides both increased capacity and increased retrieval weight - if that capacity is needed, and you don't mind the slightly heavier weight.
Don't think that large-arbor reels are limited to the expensive models. There are some fine value-priced large-arbor reels, and they are worthy options where similar standard-arbor reels do yeoman service. The Ross Canyon, Redington Large Arbor, Teton/Tioga and Orvis Battenkill Large Arbor are examples of more modestly priced reels that will work well in many saltwater situations.