this is sadly one of the liberties that need to be taken for bust artists, who are the primary ones that use the descriptions for artwork. it dumbs down the system so that the artist doesn't have a million questions on how big exactly the breasts are. it's like saying "oh, he/she's got a 13" dick," well, what's the girth, the overall tip size, the knot? what's the difference between erect/not erect? these measurements, while important, also aren't specified.
TL;DR it's universal for the artist, not necessarily the reader
True to some extent, but the equivalent for a 13 inch dick is if the artist drew it dragging on the geound. You don't need to get into specifics for some things, but if the writer says it's 13 inches, it shouldn't be drawn as if it's so long that it drags on the ground. It should be relatively right, not obviously and quite distinctly wrong.
Some of the characters are written as have small breasts, b-c cups. But if you look on the bust, their boobs are bigger than their head. That's past artist interpretation of the description, that's just plain wrong. Like drawing a red headed character with green hair. A writer doesn't need to go into the exact shade of red for the artist, but you do expect it to be some kind of red.
It's mostly Adjatha that does this, and if you look in this thread and a few other places you'll see it's a common complaint with his (gorgeous) art. His art is amazing, but that's just an area I think he struggles with.
Those descriptors are even more abstract. Hand sized, compared to whose hands? Yours? Your PC's? Well if your pc is 15 foot tall it might have gigantic hands.
I try to use a mix of those, but I still think cup size is the best system we have if you pair it with that chart I posted.
Yes, those descriptors are more vague, and if you really thought hand-sized was too up to interpretation you can go just go with the flat-chested/small/whatever sizing. A reader doesn't have to have their hand held with every little thing, and depending on how much detail is given to breast descriptions or if there are scenes that primarily involve their tiddies, a reader can get a great idea for their size and what they look like without ever have to give their cup-size.
But what I'm saying is that even with the chart system, using cup-size doesn't make sense because every character is different thickness-wise and muscle wise. That effects your cup-size. Yes, to men who haven't ever worn a bra or who has never had to actually calculate sizing it looks simple, but it's really, honestly not. Men just assuming >B is small, C is average-small and <D is big. In reality it doesn't work like that. If we had someone with a static body shape and just increased breast size, the chart would make some sense (although, in my opinion, the sizing is a little off in it) but because the characters body types are wildly different it just doesn't make sense.
If you're writing a novel, it's bad writing to say X character looked like she had these measurements and this cup size. It doesn't actually mean what men think it means, and often the numbers/sizing given would be plain wrong for a character with that body type.
Again, it's a gripe with the game but I don't think the writers are bad for including it in their descriptions. It's a very common thing in communities like this, mostly because it's largely made up of men who have never needed to really know bra-sizing. As someone who knows about the subject, it's frustrating for me and takes me out of it.