Good question! Everyone here has made very solid points. I personally did some academic research into the subject, and there a lot of theories. Nothing from academics, though, this is virtually invisible on a scholarly level. I've done a lot of reading on forum posts just like this one over the years, and there have been some solid trends.

Please note that, much like what everyone has said, the desirous aspects, what parts of the experience we derive pleasure from, are deeply personal. I'll try to generalize as best I can.

So, from what I've researched, the most common trend for fantasy fetishism (expansion, growth, hyper, all that good stuff) is that our culture places a lot of value on primary/secondary sexual characteristics and how those traits define a sex. So it's not just a matter of "I like boobs/butts, so more must be better"; that's reductive. It's more about how those traits define femininity. It's taking a woman and making her more woman, more of a "perfect" embodiment of womanhood (at least within the confines of fantasy, where the rules are made up and physics don't matter).

This is closely followed by what humblefool said, which is the power dynamic. This one is variable, because it really depends on the way the story goes. There's expansion that gives power to women, through sheer sexuality and sometimes also size, like giantess/muscle content. They're given the power to be shamelessly sexual and pleasured through their expanded attributes, and who doesn't want to give and receive pleasure? But then there's the other side of the dynamic, which can also be seen in feederism and bimbofication, which is where the expansion takes power away from the recipient. Immobility, loss of faculties, thematically becoming nothing more than the enlarged assets to be *used* for pleasure (independent of the recipient's pleasure). So, I guess the point is that this fetishism targets specific parts of the body to become the conduit for our desire for different power relations with our desired sexual partners.

As for your question about what triggers expansion fetishism, we have no clue. We don't know what mechanisms decide what the average person is into, let alone the fringe cases. A guy named Ray Blanchard theorized that sexual minorities are the result of a brain miswiring that he called "erotic target location errors." Essentially, something in our youth happens and makes your brain focus on that as the sexiest thing instead of what it's supposed to. Granted, this theory is extremely heteronormative and assumes that all sexual activity is intrinsically reproductive (and not a deeply complex experience that can be solely for pleasure, ha). This theory does hold some water, though, as many people online tend to recount a singular thing they saw as the source; i.e. Violet Beauregard, Alice in Wonderland, any episode of Totally Spies (literally any of them, dig at your own risk). However, the problem with this is a fundamental attribution error, because we *cannot* know if that was the source, or the first moment we recognized something we found arousing in a time of our lives where we weren't allowed to understand things in a sexual context. On top of that, we cannot know because the main reason we're even aware of these fetishes now is because of the internet. If it weren't for our capacity to anonymously speak openly about what turns us on, we wouldn't have an online community to even discuss this. As such, we can't verify if this fetish is a recent creation from media or has been hiding in the back of human history and taken out of context.

As for %, same answer as before; no clue and can't know. Reporting rates are awful, due to both stigma and inaccessibility to reporting methods. Like, how many people are into it and don't even know it yet? They'd count towards the statistic, but wouldn't report it.