Why aren't microphones huge in size to capture "bigger" guitar/cab sounds?

[Number Of The Priest]

I don't have much wisdom when it comes to everything-other-than-guitars, hence why I rarely post in this section, so bear with any misunderstandings or incorrect terminology I might have on this subject. Hoping to learn something out of this. Thanks.

From what I understand, a microphone and a speaker are essentially the "reverse" of each other. One converts sound in the air into a signal, and the other converts a signal into sound in the air.

The context of this question is micing a speaker in a cabinet.

The speaker can be 8, 10, 12, or even 15 inches. But the microphone's diameter may only be an inch or so. The microphone is so much smaller, so why haven't 8/10/12/15 inch microphones been built to properly "mirror" the speaker and capture the full spectrum of the speaker?

Put another way, a 12" speaker in a room sounds "big". Why do we depend on a tiny 1" microphone to accurately capture 12" of air moved by the 12" speaker? Wouldn't a 12" microphone accurately capture the "big" sound of the 12" speaker?

It's like trying to suck a Post-It Note through a drinking straw. If the drinking straw were widened greatly, to the size of a toilet paper roll, it would be easier.

[metalhobo]

Put another way, a 12" speaker in a room sounds "big". Why do we depend on a tiny 1" microphone to accurately capture 12" of air moved by the 12" speaker? Wouldn't a 12" microphone accurately capture the "big" sound of the 12" speaker?

I think you're misunderstanding what is meant by "accurate." Generally speaking a microphone with a flat frequency response is best, because then when you play back the recording over monitors with a flat response, it will sound very much like the source does in real life, which is the ultimate goal of conventional recording.

Guitar amplifier speakers are big by convention because way back when, a big speaker was the most practical way engineers could capture a good clean low-end response while still pushing the volume of air necessary to be audible in a band, which is important for a rhythm instrument like a guitar. Microphones, however, are not limited to having a loud volume nor producing a strong signal, so there are all sorts of engineering techniques used to build a microphone with a flat frequency response while remaining small. The root concept here is that with a small diaphragm, the vibrating element can be light and thus have less inertia, so it can capture high frequencies much better.

[Nightbat]

I think the easiest way to understand is to look at the size of your ears compared to the size of your mouth (and unlike audio systems our voice is limited by biological design not to damage our own eardrums when screaming)
Sidenote: the eardrums of animals that can hear better than us are in most cases smaller than those of humans

You need a large driver to produce soundwaves, but only need a small diaphragm to pick them up

[Thirdriff]

Yeah like Nightbat said, you only need a small diaphragm to record pick them up. Plus size when it comes to microphones really doesn't matter. Technology is actually making the mic even smaller. Just look at the Neumann T-120 and a few others.