Okay let me post the rule..
Proliferate: You cause an existing element to expand. This works like the generate option, except that you can either create an equal quantity of the element in the same square as its source or in an adjacent square, or cause the element to expand to fill its square (making a flame bigger or turning a twig into a small tree, for example). After you proliferate an element, it reacts to the environment naturally—water you proliferated into thin air would splash back down, for example. This affects only natural forms of the element, not durable, crafted goods.
Could an Earth Kineticist with this feat be able to quickly build a shelter for the party. I admit the roof of this structure would be problematic, but I digress.
The way I read this is this.
A kineticist holds a granite cube in their hand, and then generates another in a nearby square. Then on the next round expands it to fill the square, as I see this that would make a 5x5x5 cube of granite. Then in true Minecraft fashion the Kineticist begins to duplicate that granite cube into other squares, eventually creating a crude set of walls.
Sadly, I can find very little about this extremely versatile ability.
Isn't proliferate still limited by bulk in the same way base kinesis is? A 5x5x5 cube of granite... I don't even know how much bulk that would be
Just under 21k pounds (according to this calculator).
5 lbs per bulk = 4,200 bulk
10 lbs per bulk = 2,100 bulk
50 lbs per bulk* = 420 bulk
*A medium creature is estimated at 6 bulk to "carry [...] off the battlefield" (CRB, 272, I don't have the remaster). 300 lbs of meat and equipment is not unreasonable for a heroic-sized knight.
So... there's a lot of fuzziness around bulk estimations.