I would have thought that for bass, the bigger the speaker (15", 18"), the better the tone.
But, it seems like people here get great (better?) bass tone from a 4X10 or 8X10.
What am I missing?
The first thing to understand is that "better" is relative. Some people love 15" speakers, some people hate them. 18" speakers aren't super common in the bass cab world, and almost always have to be accompanied by other tweeters to have any detailed fidelity.
Each speaker size will have its own characteristics. As a beginner you probably don't want to hear this, but it isn't as simple as bigger is better, or more is better. Every arrangement of drivers has its own voice to it, and every cab builder tweaks the build to their own personal tastes\marketing goals.
If I had to generalize, and paint with a broad brush that people will immediately find exceptions to:
115 and 215 cabs have solid low mids and deep lows, but can be lacking in clarity. Depending on the build, some people might call them "slow" in that their larger size makes their excursion reaction time lag when compared to 12" or 10" drivers. However, some people want them for exactly that reason.
112 and 212 cabs are often a compromise between 10" and 15" speakers. They are quicker, have a good upper mid response, and can produce solid lows if engineered properly. They lack some of the "girth" of 15" or subby lows of a 10" array (410 or 810)
I don't mess with 110 cabs or combo amps. Not enough of anything for me.
I find that 210 cabs make work well in combination with other things, but by themselves are a little lacking.
To my ear 410 cabs tend to be a little scooped. Good speed and response, thick lows, but lose something in the low mids. That's why pairing them with a 115 was so popular in the 80s and 90s. That combo fell out of fashion for a verity of reasons. Some nerdy, some practical. I personally don't like how low the sit physically. The waves are hitting everyone at the knees un less you are on a raised stage. Not very helpful.
810 cabs used to be viewed as a necessary evil for just producing brutal levels of volume. And there is something to be said for that. Over the years, as the demand for smaller and lighter cabs grew, 810 builders have concentrated on the way the build the cab to give them a particular tone. An SVT 810 is something more than just 2x410 cabs stacked on top of each other. Aguilar's 810 is very articulate and clear for such a monster. Orange has found a way to get more mids out of their 810s that I think their 410s are lacking.
There are a handful of super boutique 8" array cabs out there. I have never heard or seen them aside from people flogging themselves about then here on TB.
Like everything else in the music world, cabs are a bottomless pit of information, and a financial black hole.