For everyone saying that the girl in the ad is a perfectly normal "fit" girl, here's an image of a non-photoshopped girl next to the one in the ad:
At first glance they may appear pretty similar, and they are, until you notice how the girl in the ad's sides are drastically smaller, her thighs about a foot apart, and that her arms are twigs. Does the woman in the ad have a similar shape to real women? Sure, but the problem is that it's changed just enough that it simultaneously shows a completely unrealistic figure while tricking viewers into thinking that real women look like that.
I'd also like to reiterate that I don't agree at all with the Twitter outrage but do think the ad has other problems.