I think that there's a point where trying to change things stops "enhancing" and starts to just distort things. Like a person can start looking like they're wearing a fake mask of themselves. There's a vaguely unnerving quality about it. I think this is especially true for the face, moreso than other parts of the body, because we're so strongly wired in our brains to zero in on relatively minor details in the face when we make eye contact.
Honestly, I kinda feel the same way about breast augmentation. If taken too far (and how far that is depends on how much breast there was to begin with, and the methods used I'm sure), the breast starts to take on a shape that lacks smooth curves, and looks a little bit tortured. So, sometimes I like the extra size on a model who's had her 5th augmentation or so, but the shape makes me feel like she might have been better off stopping with 4 augmentations. It's not a size issue, I make morphs which I think are sexy, and are WAY beyond any size that is humanly possible in reality. I just think that sometimes people do try to exceed what is realistically possible to do to their body with medical science, and the result suffers. I am sure there are those here who would disagree. it's a matter of personal esthetics, i'm just stating mine, not judging other personal tastes.
I share your sentiments exactly in this regard. Much as love seeing feminine curves inflated to absurd extremes (as evidenced by my choice of avatar), beyond a certain point, the allure is ruined by the obvious strain being placed on the subject's body. There are some super-sized models who wear the look quite well... when fully clothed. As soon as the shirt and bra come off, and you see just how over-stretched and - in some cases - fit-to-burst they're breasts look however, that extreme size can quickly lose it's appeal.
This is probably one of the reasons that morphs and illustrations are so appealing to a lot of people. They allow the subject's breasts (and, in some cases, their entire body), to be pumped-up and re-shaped to suit virtually any taste, without ever having to worry about scarring, over-stretching, or otherwise distorting the physical properties that make them so enticing in the first place.
Again though - that's just my personal tastes speaking. As you said, everyone has a different ideal of beauty and sex appeal, none of which could be considered objectively right or wrong. I just find that the allure begins to fade when the woman's body is clearly being strained or damaged by her own over-blown dimensions.