Absolutely. I was almost one of those people once and made life miserable for myself and those next to me. I wasn't so big that I needed two seats or that it wasn't possible for someone to sit next to me, except someone else my size, but I was getting there before doing something about it.
Having said all of that, I miss the days when the seats were wider, no, it is not me getting me bigger, and the leg room longer. There as a time, before deregulation, when you couldn't lean over and kiss your wife ... across the aisle.
Now, as many who might recognize my handle, know, I have a huge bone-to-pick with large corporations, but, in this case, it is only a small one. Consumers have a lot to do with the state of affairs inside the flight cabin and all of those intermanble add-on fees. They actually have choices among airlines, Southwest, for example, for no baggage fees and an unbelievable passenger-friendly ticket cancellation policy, to choose from. Others offer better leg room, etc, but for slightly higher prices. Consumers won't pay it; so, to a large degree, the miserable conditions inside the cabin are our fault.
To @JT, yes, I remember those days, but deregulation and competition changed all of that. While regulation stifled choice and innovation and lower prices, it did offer a lot more comfort and stable, but higher, pricing.
The more pragmatic reason is airlines base their fares on being able to fill up all of their seats. If you take up two seats, you pay two fares, period; it is not their fault you let yourself get that big, save for the few people where the cause is actually a medical condition. Then, with the proper documentation, it is an airline PR decision because you still have the two-seat, two-fare rationale.