A reel's arbor is simply the inner part of the spool where you'd first tie your backing onto.
Large-arbor reels (like the one pictured) certainly dominate the market these days. They are so prolific because they provide several advantages. Some folks (like myself) also think they just look cooler as well! Standard-arbor reels are still around as are some mid-arbor reels (which basically have an arbor sized between a standard and a large), but for the purposes of this article we'll contrast large with standard since they have the biggest size difference. So, if you're unfamiliar with the most noteworthy differences between these arbor styles, read on!
Retrieve Rate: This one comes first because it's the most marketable and widely-known advantage provided by a large-arbor reel. As mentioned, the arbor of a reel is the center of the spool. Some may even refer to it as the bottom of the spool or its core. On a standard-arbor reel, this core is very small, while on a large-arbor reel it's much wider in diameter. Think of it like a toilet paper roll (standard arbor) vs. a coffee can (large arbor). This bigger arbor helps the reel pull in more line per turn of the handle, but only increasing the arbor size isn't the right solution.
With the increased arbor diameter, a large arbor reel's overall diameter should be increased as well.
If only a reel's arbor is made bigger but not the overall reel diameter, two problems arise. First, think of a standard and large arbor reel that are both the exact same sizes overall. Once the reels are packed full of line and backing, the line circumferences and thus the retrieval rates of these reels would still be the same. Second, backing capacity would suffer greatly on the large-arbor reel since the arbor is taking up all that useful space at the center of the spool where the backing would normally be. The large-arbor reel will run out of backing before any retrieval advantage can be seen between the two.
To compensate, a large-arbor reel should have a bigger overall diameter. The larger arbor and larger overall size offers the best of both worlds: a bigger circumference for a much faster retrieve speed regardless of how much line is on the spool, along with good backing capacity. That's why if you compared a standard and large-arbor reel that were both perfect for, say, a 5-weight line, the large-arbor reel should be the bigger of the two.
Line Memory: A smaller arbor means the line and especially the backing will be wound in tighter circles which can make for a slightly more "kinky" line. A large-arbor reel lets the line come off the reel a bit more "relaxed."
Dry Backing: Many times you'll find porting along the inside and sides of a reel's large arbor. This simply means there are holes or slots here rather than a solid surface. Not only does this reduce weight by a small amount, but it helps soaked backing dry quicker by exposing a little more of it to the sun and wind.
Drag Tension Difference: As line is pulled off a reel, the drag pressure slowly increases naturally. Think about a standard-arbor reel that's full of line. As you strip line off the reel, the drag pressure feels pretty consistent at first because there's a large circumference of line still on the reel which helps the spool spin easier when tugged upon. However, as you strip more and more line from the reel, the circumference of line narrows. As you get down near the core of the spool, the circumference of line is much smaller which increases drag pressure pretty drastically. Large-arbor reels suffer from this too, but since the arbor and reel diameter are both bigger to begin with, the effects will be less.
I used to not give this much thought until I was false albacore fishing one day with my aforementioned Abel 3N standard-arbor reel. I hooked into a whopper that was estimated around 16+ pounds which took me so far into my backing it was close to spooling me. With all of that line out and getting near the spool's core, I could feel a huge difference in drag pressure—it was much firmer! I had to back off my drag to compensate for the difference, then re-tightened it as I got more line back on the spool. At first, I thought something was wrong, but nope, just simple physics!
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