D&D 5E - Fire Strom Spell- cube arrangement

We had a question come up last night with the caster wanting to cast fire storm to target large creatures with overlapping squares and deal damage twice to them. The idea was that you cast 10ft squares that in a line touching the next one. Large monsters take up 4 squares and if you set the 10ft squares so you split the large monster, you should deal twice damage. I ruled that the spell only deals the set amount of damage since the text talks about each creature in the area, thinking that the area was all the 10ft cubes, otherwise the description would say each creature in a cube. Just wanted to get other people opinion on this though. Thank you

Here is the main text from the spell below.

A storm made up of sheets of roaring flame appears in a location you choose within range. The area of the storm consists of up to ten 10-foot cubes, which you can arrange as you wish. Each cube must have at least one face adjacent to the face of another cube. Each creature in the area must make a Dexterity saving throw. It takes 7d10 fire damage on a failed save, or half as much damage on a successful one.

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