My mistake, I was calculating as if the mystic tower were size +3 not size +4, it is 2618 cubic paces (assuming 1 pace=3 yard, conversions vary), where a size+4 puts a cap of 10,000 cubic paces. On the other hand wall of thorns is 80 cubic paces at size+1, which should have a limit of 10 cubic paces, suggesting that there is a multiplier for empty space (or an un corrected errata).
Honestly I suspect the mystic tower size came from confusing diameter with radius in calculating volume.
That being said the mystic tower is +4 size with the same base of 1 cubic pace for stone and plants, while the tower Elia is describing is significantly larger and she has it as size+2.