The blocks come from dividing up the boulder. If each boulder is being divided into 4 pieces, it makes sense, but if you're dividing it up into a random amount of peices, then block weight and volume will have to be made more dynamic, and construction projects like walls and floors and furniture production would also have to depend upon achieving a certain volume of material for the product, which is made difficult now because you've got 250 granite blocks of x volume and 600 granite blocks of x*0.5 volume, and another 135 blocks of x*0.736 volume (I'm just bullshitting the numbers, but you get the point, right?) It's simpler for Toady, if using blocks in item production is made possible and for the sake of wall constructions which would have to be cahnged to account for this, to simply keep with four blocks/boulder and say 1 block = a good section of wall or x quantity of mugs.
Blocks would still be the same size as before, same volume each. The only question would be how many blocks you'd get from the stone-to-block job at the Mason's workshop. Right now, we always get 4 blocks, no matter what skill level of the mason. The rest of the blocks you could pretend that they got cracked during the process of carving the stone down into blocks and were useless.
Which means if you're willing to micro-manage a bit, you could produce about 2x the stone blocks as currently from the same number of dug out squares. The +50% to mining for using the "slow" method puts you at 37.5% chance of a raw stone, getting 3-6 blocks from that stone would give you 5-6 blocks with a skilled mason. Low skilled masons would only give you 3-4 blocks. Mid-level masons would give you 4-5 blocks.
So right now, dig out 1000 squares, get 250 stones, which produces 1000 blocks.
With a skilled mason giving you 5.5 blocks per stone, plus the slower dig rate, you'd be turning 375 stones into about 2062 blocks.