If a dwarf was sick, infirm, or elderly, how thin could they get do you think? As thin as a (normal-sized) human?
I'm asking because I want to play an elderly dwarf and that's kind of what I envision (being about as thin as a normal human).
Do you have access to Disintegrate?
Oh, wait, you want to play him.
A lot of their width is in the bones and traditionally they have wide shoulders that would remain even if they've lost muscle mass.
What you could do is give him shoulders that hunch forward and in. Much of the rest could be hidden under draped clothing. Then he'd look much like a short, ancient human except for facial features.
Or he could have a non-traditional build. Who's to say what the extremes of a fictional race might be, especially given the extremes among humankind. And being older, who knows what he's been subjected to in the magical world of Golarion. Or torture could've broken his body. Between Orcs & religious torture-zealots, there are many options for that.
As thin as you want him. C'mon, this is fantasy.
https://adscam.typepad.com/.a/6a00d8341bfa1853ef01a3fd34d527970b-400wi
If they are anywhere near Tolkien they are more likely to become obese with age, not thin.
As Castilliano said, a dwarven skeletal system is completely different than a human's - they have very broad shoulders, large thoraxes, and comparatively shorter legs with big, wide feet. Being only a foot shorter than the average human they cut quite the imposing figure, actually.
A thin dwarf would lose their thick slabs of muscle, but the shape of their body would stay the same, only their arms and torso would get thinner, as would their legs. And of course with age they could become hunched over, their spine could shorten, their hands could get warped, their legs bent, and their skin would become a veritable cobweb of wrinkles. They would also lose fat on their faces, with more sunken eyes and cheeks, and males most of all would often go at least partially bald.
All that said, this follows from data in official Age of Omens material (PF1, Inner Sea Races) - if in your campaign dwarves are different, this wouldn't necessarily be the most realistic development.
Has Paizo ever published an image of a dwarf skeleton, or anything similar?