The zoom out Spell currently has no listed benefit for the upcasting. It also seems to be relatively rare to find 5e mechanics who are able to stack with this spell, so you can zoom in / out even more than one size category, but they do exist. I would like to give a slightly easier access to this possibility.
How would the balance of the game work if we put in the following sentence at the bottom of the table? zoom out Spell Description:
At higher levels: If using a 7th level or higher spell slot, the target will grow or shrink with two size categories instead of one, multiplying / dividing its size in all dimensions by 4 instead of 2, and multiplying / dividing its weight by 64 instead of eight. The damage increase or decrease becomes 2W4 instead of 1W4.
I chose the 7th level because it's 5 higher than 2, and I've read that the levels 6-9 spells can be considered a separate level to the level 1-5 spells. In a broader sense, moving a Level 2 spell from the lower to the higher level should result in a Level 7 spell slot.
I hope the cost here, a seventh-level Magic Slot, far outweighs profit if you're two size-sized. The tremendous weight gain seems to me to be the most troubling element of this description.