No, 'sound smaller' literally means it sounds smaller - as in size, a measurable thing. Or take it to mean it sounds like an amp with a smaller speaker if you want...either way it's a palpable and measurable descriptor. Boxy, well, that even has a defined meaning:
I'd understand if we were using dumb TGP-esque nonsenses like 'mojo' or 'creamy' etc but we're using definable terms here.