You are precisely wrong here, echoes require open space to proliferate.

Go out to a field and try to produce an echo. They literally require walls to bounce off of.

Isn’t the reason you are invoking a contortion of scale to shift our focus to inside one of these smaller bubbles/cells motivated by a desire to induce a sense of some small degree of open space around us? In a sense, aren’t you arguably still invoking the idea that space is what allows echoes rather than density and enclosure?

You need some space yes, ideally the inside of your chamber needs to be mostly empty and insubstantive.

However, echo chambers can not be filled with too much space, because echoes don’t work at infinite scale. Sound dissipates and loses energy as it travels through air, so for an echo to occur and you to hear it, you need to be a relatively short distance away from a wall. To be truly echoey and hear multiple echoes of the same sound bouncing back and forth on the walls in front of and behind you, you need those walls even closer together, for not just the extra distance travelled, but also how much energy is lost during each reflection.