It's no problem, it's not against our rules: Works that used this stock as references, which are original content and not copies of these stock images, can be distributed commercially and without attribution, even if they would otherwise fall under the definition of being an "Adaptation" as per the license.
First, the item you're commenting on is female hands, just in case you were thinking it was male. More over, at least some of those pictures have hands much closer to the camera than other parts of the arms, given that this is, you know, a perspective set - i.e. one with things either approaching or going away from the point of view of the camera - of course the hand will look bigger than the arm behind it - the forearm is either completely hidden or much less visible in the majority of those shots, and the upper arm is about a foot farther from the camera than the hands on average. Maybe this wasn't obvious to you, but even if it wasn't we find the line of thinking in your comment to be problematic.
What follows is far more blunt (and if you were actually being playful/joking, then maybe this doesn't apply to you, but then you ought to communicate your meaning better).
Frankly whether or not you find it gross is up to you, but as far as we're concerned that's a fundamentally flawed way of looking at people, the world, and attractiveness. Preferences are preferences, and we wouldn't hold them against people, if they are kept in their proper place. But you made a normative claim like 'x needs to do y', and from context it's reasonable to suppose it's because you really believe that people should strive to conform to what you or your culture finds attractive. Screw that - value judgments like that need sounder backing behind them than the fact that something looks unappealing for someone. If and when we work out, or do anything, it'll be because either A. our ethics dictate it, B. it contributes to our health, C, we enjoy doing it, or D. we actually literally have to. It certainly won't be because some twerp on the internet didn't like how we looked.
TL;DR: Every time you deprecate someone's appearance, you make the world a worse place. Far more harm than benefit comes by mindsets that judge by appearance. Intentionally contributing to that is fundamentally unethical.
I don't really care what this person looks like to be honest. I also wasn't being serious on a level where it should offend anyone, but I am seriously suggesting you get another female to do more realistic looking poses because this lady is super slender and yeah I kinda thought it looked gross. Its just an opinion, though, because I don't think everyone feels the same as me and I don't think they should either. I do notice the angles and perspectives, but my original opinion stays the same. I don't hate or like this person because of their "appearance", I don't really like the stock. Normally I wouldn't have even stated my opinion but it was in the top 24 hour or whatever and I was confused about why no one had brought this to attention.
Whether or not you judge the person as a person based on their appearance, you still seemed to think an offhand comment about how we should modify our lifestyle was appropriate. We're both mentally mature and secure enough that that doesn't affect us negatively, but when people make comments like that as if it's perfectly fine, the side effect is that those in the middle of their development as human beings tend to see such judgmentalism as perfectly fine, and simultaneously tend to develop more insecurities. Consequently those most vulnerable to being harmed by self-esteem/self-image issues (and there ends up being more such people as a result of the above) tend to get harmed more in such societies.
Anyway: I would wager that she is "super slender" only relative to populations where food is unnaturally readily available to a large portion of the populace (unnaturally in context of the bulk of human history). In fact, realistically, she is probably average as far as what you see in the stock we've made of her for humanity as a whole, and still more fleshed out than many people in the world ever see, let alone become.
As for the stock, we're not forcing you to use it. There's other stock out there with female limbs that match your needs more, most likely. And it's not like we're a professional outfit with high-end equipment: you could probably snap similar quality pictures yourself of some willing volunteers (or by yourself - the female model does the vast majority of her own hand/feet pictures when she has a few spare moments) just as well as we can. While we wouldn't be against adding another model if someone we knew felt like doing it, we're not going to go out of our way to find one.