I think the ability to steady fill them up makes no limits.
It's not like having a fixed number of volume.
So a lot of women will go way larger with expanders than they had in mind when going for some fixed implants.
Maybe that is also one point why some went for another set of bigger ones. For example going for 500cc and after they become familiar with the size, they thought maybe 800cc would fit better.
So with expanders they don't have this limitation and have easy access to fill them up, this makes it easy for those who are really into them cause of boobiegreed.
And another benefit is, that they can slowly stretch the skin and not going in huge steps. So their skin have enough time to grew with the new volume.
And it always depends on the point of view, one woman with maybe some A-cups and a slim body would think that going for 800cc might be a huge number, but after some time and becoming familiar with those 800cc she might answer positive to the question if she could imagine for going larger.
So it is not in general, but if you look at those woman with relly huge implants what they had before those, very often they were at the beginning of the alphabet with their cups.
So given some woman with boobiegreed and solid A-cups some expanders will end up in another beshine 
I remember a survey done perhaps a decade ago where almost 60% of women reported that they wish they had gone larger after getting implants and that almost 25% said they went for a second operation to achieve a larger size. I feel they are often sold on an undersize based on the surgeon's recommendation and a fear of looking too large.
I agree with your assessment; it is seemingly always those with boob envy that seek to go larger than average with implants, yet, there are those with very large natural breasts that also possess boobie greed, wishing their natural breasts were insanely huge. But when we are talking implants, those experiencing boobie greed always seem to be those that started out as A cups. Even CC, a natural D if I remember right, was not looking to be as huge as others, seeking 2500cc implants rather than going for the largest.
But there is something about having the freedom to expand easily that almost always pushes the expander girls far past any initial expectation. Maybe a subconscious urge was already there, which is why they opted for expanders to start, but once they start expanding, they always seem to exceed 2000cc. And I find it interesting that while many big boob stars of the 90s remarked that they didn't want to continue living with boobs that were frequently in their way for doing things, it seems that the expander girls start to embrace the ways their boobs make them do things differently, turning a negative into a positive. And as Beshine said perhaps 10 years ago, she loved experiencing those things so much that she couldn't wait for her boobs to get even bigger and create new big boob problems.
It does take a certain attitude to have implants of a size where you stand out in a crowd, but I don't want to cubbyhole that as being singular. Foxy loves the attention her boobs bring her, thus the low cut tops; Beshine initially wore sexy, often braless but not low cut tops, but within a couple years, always dressed modestly and attempts to avoid drawing attention to a certain degree (as if that is possible). There are a number of expander girls now past 4k, like Mary, that also have diverse views of how they want to look in public and while I only see what is shown, Mary seems to take a different path of very sexy and tight clothes, usually without being too revealing; a definite sense of modesty, the I'm not bad, I just built that way. I do love the girls that seek to be the girl next door, albeit with huge fake boobs.
Some find their happy place along the line, whether it be 4k or 6k, but a few like what they feel so much that they are refusing to offer goals, as Foxy once did, only to repeatedly move the goalposts, to now just wanting bigger. I don't think any realized so early on as Beshine that they wanted immense boobs, which is too bad because much of achieving your dreams requires advance planning, like how likely is a surgeon to operate when you are already past a certain size and how to not experience Jessy degree sagging with heavy implants; these things cannot be addressed retroactively like Martina once thought (and then suffered a setback because of that naivety), and like Foxy has now discovered. I can't help but wonder, if Brittany had the knowledge and insight to have a mesh placed first, would she now have her 5000cc boobs and be thinking larger rather than stuck at 1k? You have to make smart choices, choices not every surgeon offers up front unless you ask. It may cost a bit more, but when you can have 20k boobs and not worry about sagging, dehiscence, or your boobs pressing on your diaphragm, it is money well spent. The problem is, you don't always know you have boobie greed until after the fact. Mary has not set any goals that I know about, but I would like to hope she is planning with good advice and information.