DVC rooms are getting smaller because, even though it's practically mandatory to complain about not getting enough out of the rooms (amenities, housekeeping, theming), the trend, on the party of purchasers/owners, seems to a desire to squeeze as many warm bodies into something the size of a closet to save money and use fewer points. Building rooms that are 10 or 15% larger isn't going to demonstrably change the cost of building a new tower/resort or in turn purchasing a DVC contract. We're not talking about trying to fir as many staterooms as possible onto a cruise ship, where space is somewhat finite.
People would rather have a studio that sleeps 7, over a villa that actually has a kitchen. Disney knows how quickly studios are reserved, and conversely how long larger villas take to book.