The slight DM pedant in me is torn about how you chose to interpret the size of the sphere.

There is absolutely no reason to not use volume as the thing being modified by the item EXCEPT as far as I am aware the RAW is that the fireball is measured by its radius.

By that reasoning the new fireball should have a radius of 200 ft. Which is obscenely large and also puts the caster within the blast radius.

If I remember correctly in older editions the fireball would also fill up rooms until it's full VOLUME was achieved not necessarily just it's radius.

Meaning a 20ft radius fireball would fill a 5ft x 5ft hallway/tunnel (weird size I know... easy math) for 1,340ft and the new 200ft radius (33,510,300 ft^3) fireball would fill the same hallway for... 1,340,412 FEET OR JUST SHORT OF 254 MILES OF TUNNEL!

If my players were stupid enough cat fireball on a magic enhancing object, personally, I'm using the later object and also multiplying the damage by 100.

Their new party can start by investigating the massive village size crater recently discovered by the locals.