Here's a tale of two spinning-reel spools. The spools shown in the photo are from reels of similar size. One is short and wide. One is tall and narrow. So why is there a difference?
Wide-spool spinning reels recover more line with each turn of the handle. That means there's slightly less line twist created than with a narrow-spool reel of equivalent size. Wide-spool reels enable longer casts, too. That's because each line wrap coming off the spool during a cast is slightly longer, which ultimately reduces friction on the outgoing line.
I once got curious about this, and so measured casting distances with both narrow- and wide-spool reels. The set-ups, casting weights, and line were otherwise identical. A wide-spool reel enabled casts that were roughly 20 percent farther.
On all spinning reels, the spool moves up and down as you crank so line is spooled evenly. A wide spool isn't as tall, so it doesn't have to move up and down as far. That allows for smaller gearing inside the reel, which in turn allows a smaller and lighter reel body.
For all those reasons and more, wide-spool spinning reels are one of the most important recent trends in spinning tackle. U.S. Reel pioneered this concept in its modern form. Now there are some copycats. Bass Pro Shops adopted this concept a few years ago for a line of its spinning reels. (I should point out that when I asked them, Bass Pro vehemently denied copying anything.) And I've just noted that the recently rejuvenated Lew's brand has come out with a wide-spool spinning reel too.
Wide-spool spinners are still much less common than reels with tall and narrow spools. That might or might not change eventually, but for the time being, at least, it's nice to see something different. And, I think, better....