I don't believe that I implied it was. Hence why I chose the words "I submit that..."
Not true.
The Rules do not pretend to be exhaustive, nor do they attempt to be (nor could they be, realistically). So they give "a rule" within The Rules, that "...As a rule, any disability purchased as a Flaw at character creation is part of a character's Essential Nature, while disabilities acquired later are not." (p 80, par 1) So the Rules are also saying that any Flaw that affects the human form is part of EN.
If Small requires a virtue and thus is EN, then it stands to reason that Large would be as well There is no good argument that shrinking a person redefines EN but enlarging them does not.* Hence "normal size" is another part of EN.
[i](* A predictably simple quibble would be that the Rules only mention "Flaws". I believe they do this only because, in the context of that paragraph, the previous text (p 79-80) describes losing body parts and going blind - hence the example is that a blindness caused in-game is not part of EN, but a character that starts with the Flaw must endure that as originally designed. The authors are concerned about trivially removing Flaws with magic, but the parallel for gaining Virtues is implicit.
If anyone is going to explain this away, they must first explain why being small is EN by the above rule, but being any other size is not. And why a mage can enlarge a normal person to size +1, but by the above rule that same mage somehow cannot enlarge a size -1 to "normal". )[/i]
As far as game-balance, that is a solid argument. If you're going to allow it, that'd be the way.
Ummm.... while being poor could, in some loose sense of the word, be considered "a disability", I believe it's fairly clear to most that they're talking about physical disabilities. Being an Outlaw or having Monastic Vows could be seen as social disabilities, but that's not what this discussion is about. Really.