I do remember him saying, back when 5e was still in playtesting, that he specifically wanted big, splashy damage spells to be the most effective, because they feel the most fun to use, and that his intent was to design the math such that the character optimizers would be able to pull out their abacuses and show that, yes, fireball is the best spell. That was a pretty general claim that I could see meaning they designed those spells to be at the peak of what their math allowed while others were more average. But I could have sworn he later made the more specific claim that how they ended up achieving that goal was by figuring out how much damage a spell of a given level should do, and then giving those spells one or two more damage dice above what they should do for their level.