dnd 5e – What is the "upper limit" for the size of an object that can change True Polymorph?

To change an object to a creature, the player manual says:

Object in the creature. You can turn an object into any creature as long as the creature's size is no larger than the object's size and the creature's Challenge Rating is 9 or less. The creature is friendly to you and your companions. It affects every one of your moves. They decide what measures are taken and how they move. The DM has the stats of the creature and resolves all its actions and moves. When the spell becomes permanent, you no longer control the creature. Depending on how you have treated it, it may remain friendly to you.

So my question is, what's stopping a player from turning a mountain into a goldfish? Are there rules or restrictions on spells that I do not know about?