In my mind, a McMansion's defining characteristic has always been that it's far too much house on far too small of a plot.
It has pretenses of elegance that are betrayed by a gaudy claustrophobia. It's like loudly proclaiming "LOOK HOW MUCH HOUSE I HAVE," while generally sticking out like a sore thumb from the other houses in the neighborhood.
They are far too much house on far too small a budget. They are about maximizing those wealth-connotating features of a home which are comprehensible to the upper-middle-class laypeople who buy them: raw square footage, bed/bath count, "classical" architectural features (poorly executed, because the intent is to check "expensive-looking" boxes, not actually to execute a quality building).
This same style is very consistently-expressed at differing scales in different housing markets across the U.S.; the lot size varies with land prices but the ethos is the same.
Is that second paragraph unfair though? You buy/own a block and build a new house. You know there are a lot of useful things you need or might need in the future (garaging, bedrooms for future kids, etc) but your constraints of land size and budget remain so the result is often the efficient answer.
It has pretenses of elegance that are betrayed by a gaudy claustrophobia. It's like loudly proclaiming "LOOK HOW MUCH HOUSE I HAVE," while generally sticking out like a sore thumb from the other houses in the neighborhood.