The enlarge reduce spell currently has no listed advantage to being upcast. It also seems relatively rare to find 5e mechanics that are able to stack with this spell, allowing you to grow/shrink even more than one size category, but they do exist. I would like to allow slightly easier access to this possibility.
How would the balance of the game be affected if we were to add the following phrase at the bottom of the enlarge reduce spell description:
At Higher Levels: When cast using a spell slot of 7th level or higher, the target grows or shrinks by two size categories instead of one, multiplying/dividing their size in all dimensions by 4 instead of 2, and multiplying/dividing their weight by 64 instead of eight. The damage increase or reduction becomes 2d4 instead of 1d4.
I chose 7th level because it is 5 higher than 2, and I have read that the level 6-9 spells can be considered a separate tier of play to the level 1-5 spells. Thus, in a broad sense, moving a level 2 spell from the lower tier to the higher should result in a level 7 spell slot.
I am hoping that the cost here, a 7th level spell slot, far outweighs the gain of being two size categories larger. The huge weight increase to me seems the most concerning element of this description.
Best Answer
Creature size changes (by two steps) can become problematic
The damage increases aren't the factor that I was most concerned with. What raised my eyebrows was the change of creature size catagory by two opening up mechanics that would never have been possible before.
Levitate
The first thing that came to mind for me is the levitate spell. This spell normally has a weight limit requirement of 500lbs which generally means medium size creatures according to this question.
Allowing a spell to reduce a Huge creature to a Medium may (upon DM approval because weights really aren't defined in 5e), opens them up to levitate as a control option where that normally wouldn't be the case.
There may be other spell mechanics that could be made available as well, but this was the first one I thought of (and it's pretty powerful.)
Grappling
This is another place where PCs normally couldn't engage in a grapple with creatures of the Huge size. There are even builds with races that allow you to act as a larger size that would then enable you to grapple what was once a Gargantuan creature. Grappling is generally underwhelming so I'm not rating this as a major problem, but it does need to be mentioned.