perfectfire, is the point to all this that you grew up in a really small house, or that you currently have a really big house? I don't quite get it.
EconE is right about the wants vs needs thing. There are several examples out there of people living in "microhousing" solutions (less than 300 sq ft for two people, for instance).
Regarding your complaints about my argument of utilization, please allow me to elaborate. Current utilization is set by both the actual *needs of the resident and societal norms. You need to eat, the societal norm is that you purchase groceries on a regularly spaced schedule and then prepare the meal at a later time, ergo you need a kitchen regardless of how many people live in your house. Other societies operate differently, but their communities are also structured differently. If the expectation is that nobody has a refrigerator (many poorer areas), then you will live near (within walking distance usually) of a market where fresh meat can be purchased the day it is to be used. In Europe, for example, many people live in smaller homes, but they also tend to have small local grocery stores so that on the way home (walk from subway perhaps) it's convenient to stop by and pick up a loaf of bread or a carton of milk. If that's the world you live in, then having a fully stocked American style kitchen is indeed wasteful, but it's really just a different norm.
That's why I won't berate a single who buys an 1100 sq ft house. As was pointed out earlier, his only other SFH option was a 950 sq ft house. Sure, you can argue that he doesn't need a SFH at all, but then nobody actually does. Owning the ground under your feet is something our society values (there I go, talking about societal norms again) and so it's unsurprising to find a person within that society who has the same values. Particularly if they have concerns about the community costs associated with the alternatives (HOA dues, emergency funds raising votes).
Which brings us to why McMansions are worthy of such scorn, even if five whole people are living in one. The premise of a McMansion takes many of the least agreeable aspects of our society and emphasizes them to the breaking point. Many people are uncomfortable with the rat-race American society has become, but owners of McMansions seem to be saying "look at me, look at what a big rat I've become!" Second, the McMansion emphasizes the loss of community that many Americans feel. Forty years ago, it was common for people to spend much more time outside their homes, which meant they got to chat with neighbors across the fence. But when you put a 4,000 sq ft home on a 5,200 sq ft lot, you don't even have a yard anymore. You're ostensibly saying to the whole world "STAY OUT!!" Finally, the McMansion pulls off the feat of combining what many people consider to be the worst aspect of living in a city with the worst aspects of living in the suburbs. It creates a community as densely built up as any multifamily living situation (condos/townhomes), but at the same time you're still clogging the streets to get to work in the city centers. Most people probably can't put it at words, but those are at least some of the essential reasons McMansions make us sick.
You can't really apply any of the same arguments to a single guy in a 1000 sq ft home. He's just a family guy who never actually got married or had kids.
* note, "needs" for this conversation will relate to those things which for the price make a person's life more comfortable rather than less.