Answer by Thomas Zerbarini:
There's an ongoing debate about this very issue by many passengers that find themselves competing for space with a Customer of Size (COS). Many US airlines have a COS policy that requires a larger passenger to purchase two seats if they do not fit in a single seat. From what I understand it it difficult to enforce this policy because most passengers purchase their tickets on-line or over the phone. It's also a very delicate matter for gate agents to confront to enforce, so the policy tends to get overlooked or missed.
There's a good article from the UK that analyzes this situation that you may find interesting…