We don't have 6" subs for many reasons. You would need a lot of them for the output people are looking for. More drivers would mean more coils, which would equate to a greater impedance rise. Less power out of your amplifier. Not for many reasons. Only for the simple reason of why use many smaller subs to achieve the same volume output and low frequency response of what you could achieve with a single big sub.
Also, most people have a decent amount of room for a box.
You can play lower louder, but a 12 and 18 can both play just as low..
Maybe almost but not quite as low. But more importantly, it's the frequency it can play at vs the SPL volume...
It's the same analogy but when comparing a super small subwoofer to a massive one, the difference is huge.
I've ran many sizes. The reason I'm running 15's is not to play lower, but to have more output in general.
My SA8v2 is the lowest playing setup I've ever heard. Box tuning was in the 20's. Was it the loudest? No.
Correct, because it isn't a 12, 15 or 18 inch.
Box tuning is more important than cone size, to a point. I'm assuming you've heard good home audio subs that are small. If not, you would be surprised by how low a 6" driver can play in a horn enclosure.