While you are correct the Paladin would most likely use lower level spell slots on low hp creatures and higher level spell slots on high hp creatures. So in the end I think it about evens out in regards to excess damage, maybe with a slight nod to the Fighter.
It does vary depending on whether you are fighting lots of minions (fighters and area effect spells inflict more damage) or big bads (high damage one-shots like smites and single target daily spells do better.
I can't speak about single classed fighters because the fighters in my group both multi-classed at 5th and 2nd level but I will say that the (bow) ranger and (trickster) cleric came out as lower damage than the Fighter/ranger/barbarian and fighter/rogue/warlock in the tests I did but there really wasn't a lot in it once you only counted the damage that mattered.