Sexual attractiveness is a very complex matter, as you're probably aware even if it feels or seems alien to you.

Different people have different tastes and whether it's something as mundane as hair or eye colour or height or something as controversial as weight or gender were still not completely sure how and why it comes to be this way.

Some of it's innate, from somewhere in our minds, genetics perhaps, and some of it's learned, from what we see around us and what we see others being attracted to. Then there's the fear bias, we pretend to be something we're not and pretend to be attracted to people we aren't because we fear the consequences of honesty.

One factor has to be our evolutionary history. We might be sentient, mostly rational creatures capable of managing and overriding our instincts (mostly but not wholly) but we came from ancestors who weren't so capable.

For them and for other such animals today sexual attractiveness is purely a matter of the reproductive drive. It's an oversimplification but animals seek to mate with other animals who present the best available way of passing on their genes. Mating displays and outward appearance in general are critical in assessing this.

Simply, healthy individuals are better repositories/donors of genetic material than unhealthy individuals are. Simply, fatter/chubbier people are physically unhealthy when compared to slimmer, healthy people. Hence slimmer and/or athletic people tend to be more physically attractive to us.

It's not a hard and fast, unbreakable law. It's a scale. It's not the only one by any means. It's the same with people who are too thin, anorexic, whatever. They're simply not healthy and deep down we know this.

Media bias confuses the issue, especially from the fashion industry where models are not successful based on how they look but on how they make the clothes they model look. Thinner, nigh-anorexic people offer an easier frame for designers to work from since they don't offer as many 'unconvenient' bulges and curves to accommodate. It's easier to design for shapeless people. In turn this shapes public perception.

That's where I believe the slimness/petite aspect comes in. As for the busty aspect, male animals would look not only for general health in a female but also for her ability to care for the offspring. For mammals this might manifest in the size and capacity of a female's mammary glands.

For animals it's all subconscious and though we humans are sentient we're still animals whether we want to admit it or not.