McMansions are big homes that are also sometimes built close to their lot lines. So much so that one Florida resident suggests you might know what your neighbor is up to:

I’m referring to the handful of gargantuan McMansions cropping up now, and, insult to injury, all seem to hug their property line so closely as to case a gray shadow on their neighbor and to heighten the anticipation of hearing the next time your neighbor flushes his toilet. Do these owners have 35 grandchildren to bed over the winter? Or are we seeing the beginnings of a contest to see who can build the biggest and gaudiest house on the island? Nantucket, here we come!
The idea that you can hear the flushing of a toilet from next door might be a bit of an exaggeration. Put this next to the 35 grandchildren and some creative license might be at work.
But, let’s say there might be some truth to this. Are we talking about McMansions within five to ten feet of each other? There may be some windows or doors facing the neighboring home. If it is a quiet neighborhood, house noise might travel. Residents of single-family homes tend not to like noise from nearby (examples here and here).
If it is a sizable home, could a neighbor identify which bathroom is the source of the noise? A home of over 3,000 square feet could have numerous restrooms. Perhaps some are located closer to exterior walls. Perhaps some have louder commodes than others. Perhaps some bathrooms are used more regularly.
The point still holds: some of the concern about McMansions is not just their size but rather how much of their lot they take up. A large home that covers a higher percentage of its lot and/or is very close to a neighbor will appear even larger.