There are distinct differences (at least in my mind) between mansion, McMansion, and a large house. The most obvious ones being that the McMansion is typically a veneer of architectural bling over shoddy construction, and the stereotypical 5000 sq ft house on a 10,000 square foot lot, in a development of similar houses crowded cheek to jowl, so that - if so inclined - you could probably piss out your bathroom window and hit your neighbor's toilet. Your true mansion is much higher quality and on significant acreage*, while a simple large house eschews the bling.
*And probably has walls & plantings so that passers-by will at most catch glimpses of the grounds, while the point of the McMansion is to be seen. Indeed, it almost always has landscape lighting arranged to show off the house at night.