With increased size comes increased weight(generally speaking). If you were to simply scale up the size of a woofer, you would also need to scale up the strength of the parts to be able to support/control the larger moving parts. You would also need to scale up the size/strength of the motor assembly to be able to control the movement of the cone and control it properly.
Look at the 34" audiobahn/cadence woofer. In short demos it worked, but it's practicality for everyday was just nonexistent. In order to make the cone rigid enough it was made heavy. The suspension and spiders couldn't support it and after a few months would start to sag significantly if mounted face up.
Look next at "The Bomb", Richard Clark's 5' woofer in a bread truck. This thing was built as a one-off, one-of-a-kind "speaker" to displace insane amounts of air to meter high. It's first design to see competition ate itself early on and had to be rebuilt. The frame, in this case, could not support the forces being exerted on it by the ridiculously powerful motor and substantially large cone assembly.
I'd say even the 21" you mention is not an everyday woofer. You really don't see more than a handful of manufacturers making them and most of them are in pro-audio setups where excursion is not as extreme as car audio.
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
Yous just figures that out? All the hots ones is crazy!...And deh ugly ones too. - Toki Wartooth