After I remembered reading it somewhere, I tested burying spheres of the same type on top of each other (same spot) and the result was interesting.
Let's say you have multiple red spheres, sizes 5, 10, and 30 (the following are not exact figures): if you buried the size 5 and then put the size 10 or 30 on it, it would grow a bit, most likely becoming 7-9 in size. Now, if you buried the size 10 and added the 5 or 30, it would be about 12 or so. If you buried the 30 and added the 5 or the 10, it would become 33 or so.
What this means: if you burry the largest sphere first and add multiple smaller spheres onto it, it could reach a very large size. Also, don't add two big spheres together; the result will only be slightly higher than the first sphere buried.
That's as simple of an answer as I can give, I hope it helps.