I think the real question is: is being fat the fault of the individual or are there cases when it cannot be helped (such as a genetics). I don’t know the answer, but here are two hypotheticals:

If it is the fault of the individual then he or she can willingly gain or lose weight according to any aesthetic, or health, value he or she may possess. Any argument in favour of one or the other ultimately must settle with individual preference.

If it cannot be helped, such as a genetic determination (e.g. metabolism, hereditary), then weight discrimination is unjustified. Yet, as technology advances, even this biological argument diminishes.