There seems to be something of a fad lately, not so much with artwork depicting big-breasted women, but in the reactions that people have to it. I'm not sure if this is recent, or if I just noticed, but it seems to be everywhere now.

All of a sudden, it seems to be "cool" to hate artwork that depicts women with big breasts.

The reasons for this seem to be three-fold - though most postings against such artwork tend to blend these points together - and I'll go over them here.

The Personalists

Some people don't like big breasts in artwork simply because it's not their thing. In this case, they're simply stating that the depiction isn't to their personal taste. The same way some people like tall women, or blonde hair, or Asian girls, some just like smaller breasts.

People who speak out against chesty women in artwork for this reason tend to be slightly less nasty about it than people who don't like it for other reasons. It's more "hey, I like your art, but she'd be hotter if her boobs were a bit smaller," than anything else. That said, a lot of these people still tend to come off as selfish, often phrasing their statements as though they expected the artist to draw something that caters to their specific tastes.

People, if you didn't commission the artwork, then don't bemoan the fact that it's not exactly what you'd enjoy!

The Realists

These people tend to dislike big breasts because they find such things to be unrealistic, at least to some degree. While there are examples of women who naturally develop very large breasts, these are the exception rather than the rule. More than that, however, these people tend to think of the depicted woman in very literal terms. They see artwork of big breasts and think of the back pains it'd cause, the sag they'd be subjected to, or how they'd simply make daily life a pain in other ways.

None of these points are wrong, ask any woman with an exceptionally large chest and she'll tell you all about them, but none of that is the point. Artwork isn't mean to be taken so literally. Big breasts are representative imagery of feminine sexuality; a woman with huge breasts is meant to evoke the impression that she's meant to be a sex icon (to some degree). People who point out that such things are ridiculous for how unrealistic they seem aren't understanding that they're not meant to seem real - they're meant to convey an idea.

The Activists

These people tend to rail against most artwork depicting women in a sexual aspect because they find it to be denigrating to women. Specifically, they understand that big breasts in artwork are meant to showcase a feminine sex symbol - they just also believe that doing so has the effect of reducing real women to being nothing more than sex symbols also. In other words, they find all erotica depicting women to be an attempt to make all women into nothing more than erotica.

Of course, this is ridiculous for several reasons. First because it assumes that people are empty vessels waiting to be filled with whatever imagery comes along. Just because someone enjoys erotica doesn't mean that they lack the ability to distinguish that (as a fantasy) from real life. Secondly, because it assumes that the intent of the depicted woman is to try and reduce real women to a similar, symbolic state. This is foolish first because most artists simply don't create their work with that goal in mind, but even if they did there's no flawless message for transmitting that message through an artistic medium; people will read into whatever message they discern. Finally, it assumes that even if erotica depicting women does have an impact on peoples' perceptions of actual women, that impact must be negative. There's an entire branch of feminist thought, called "sex-positive feminism" that takes the viewpoint that pornography (and erotica) is actually beneficial, both for women and for men.

Summary

As I mentioned above, people often speak out against big breasts in artwork for a combination of the above reasons. Someone might talk about how they don't like big breasts, and they find artwork emphasizing them to be denigrating to women in general, for example.

However, for a lot of people, I don't think any of the above is the real reason why they want to speak out against such art. I think that for a lot of people, they simply want to look righteous, and think they can do so by speaking out against something controversial. For them, it's not about any particular opinion they have, but about the opinions they want other people to have of them.

And that is the worst reason of all to talk badly about the art that someone else worked so hard on.