What makes a Leviathan scary is not simply its size. Many things can be large without eliciting fear. And it’s not simply that one could gobble you up in one bite, oh no. Being eaten is terrifying in its own way (digested, doubly so), but no, that alone is not what makes the skin crawl and mind race.
It is context: that which you cannot see, the inscrutable depths of a world you cannot hope to navigate as you are (or possibly will ever be). There is a sense of helplessness — surging panic — out in the deep waters. Will the Leviathan be friendly or malicious? Will the Leviathan even care to notice you as it swallows you whole?