Sub base for slabbed area with cabin on it
The best base has a range of aggregate sizes so that they all fit together, lock in place and fill all the voids tight. Therefore the answer is dependent on the size of the pieces you break the old slab into. The smaller, and closer in size to the rest of the fill is best. Or have adequate space between larger pieces so that you can be sure to fill all the voids very tight or over time the soil and other aggregate will migrate to the voids and settlement will occur.