So cho size doesnt scale with hp, it scales with ult stacks.
Zac shrinks / grows by CURRENT hp as a result of his theme being essentially combat-flubber and as such damaging him would make sense to reduce his size.
Issue with naut being bigger:
Takes more skillshots for his team = better frontliner so might get nerfed.
More unit collision issues (creep block etc)
His anchor grows with his model creating clarity issues for hook range / width, aa range (measured from center), e range (measured from center)
Why, thematically, should he be so big? in his lore he was a sailor who dove in a diving suit and got magically sealed in by an unknown power. Nothing even points to him being any taller than varus or a normal size person.