Acoustic instruments rely on their size and shape for their acoustic properties. Reduce the size and you reduce the volume or alter the sound.
Most electronic instruments, on the other hand, have become smaller than their acoustic counterparts. Drumkits, keyboards, guitars, violins, just to name a few. While acoustic properties still matter in many electrical instruments, they’re not as important as before. I’ve seen an electronic cello that was pretty much just a plank with strings.
Which instruments?
Also: laws of physics for some of them.
I am speaking broadly about the majority.
I’ve been pondering why, despite the numerous technological advancements, they have yet to reduce in size.
I’m no expert but I’d say it has. My Casio Privia keyboard is way thinner than a grand piano
It has? Take a synthesizer from the 1960s versus one from the current year, for example. And, while this is only tangently related, the average laptop can master music just as well as a massive counter unit from thirty years ago.
As for acoustic instruments, there are two things to consider: first, the sound of an instrument is dictated by its shape; change the shape (size), and you change its sound (not the same instrument anymore). Second, music is a largely tradition-worshipping community. People want a big, old-style guitar because it’s what their idols played with.