If what I recently read is true, McMansions are no longer popular and smaller homes are becoming more common, so the “the speakers are too small for the room” issue will be less common, too. However, matching the speakers to the room size has always been an important requirement, if the best performance is to be realized. You can’t very well expect a pair of 4” two-way bookshelf speakers to fill any room, never mind one that’s 20’ x 30’ x 12’ and if they can, kudos to the designer. Scale down the dimensions for both and you have, well, Bose. It’s a lot like the “teaching a pig to sing” analogy (it sounds bad and annoys the pig). OTOH, 15” woofers in a bedroom isn’t usually a good match, either. In an extremely large space, it’s similar to designing a PA system for a live music venue- the speakers must provide the energy needed to reach high SPL without being overkill, the amplifier power must be sufficient and the system must have some headroom. How much depends on the dynamics of the music. If the room is acoustically treated, it may require more power/sensitive speakers, depending on the music. If not, it won’t sound great at high SPL because of the reflections.
As far as people not using more than a fraction of the power- that’s easy to assume if the person in question buys a receiver or amplifier that’s at least adequate for the application but when a tiny budget is part of the equation, it’s pretty easy to try the equivalent of towing a city bus up a steep hill with a VW bug- it may get there but it’s not going to be pretty and you may see parts flying through the hood. My first receiver (Pioneer SX-525) may have been rated at 17W/ch at whatever THD percentage but when it was tested at the McIntosh amplifier clinic (hey- they said anyone could bring in ANY receiver or amp), it pushed a blistering 13W/ch. OTOH, the speakers I had were fairly sensitive. They sounded like crap, but they were pretty sensitive. Since the system was in a small, live-sounding rec room, it was loud enough that my parents would bang on the sides of the clothes chute to get me to turn it down. SUCCESS!
Personally, I think manufacturers need to do a bit of destructive testing because they will never know how much power or abuse their products will take and we all know someone who treats their stereo like a farm animal, right? I also think speaker manufacturers need to place their products in rooms of different sizes and acoustical properties, in order to get an idea of how bad they can sound. If they only use rooms for listening tests that are very good, acoustically, it’s hardly a real-world scenario because a corporate facility doesn’t have to deal with WAF or an interior designers’ “vision for this space” and that usually means "In this space, I will see no speakers".
BTW- asking someone’s girlfriend for a lapper is only a problem if she shrugs, says OK, and enjoys it.
