I know it's a troll, but I actually agree, I would dump a girl if she ever got fat so I would expect the same if I were made redundant or whatever.
getting fat is not irreversible; if you are not there for your loved ones when things aren't good for them (i.e. only when things are rosy), then what's the point? anyone can be nice and supportive when everything is perfect, but when the going gets tough is when you really find out who you are.
[although i would concede that this generally makes more sense in a parent-child context, where obligations are owed by parents to their children, and perhaps less so for a bf-gf relationship (but post-marriage would be a closer analogy)]