This is trivial, I'm surprised it took me this long to internalize this fact.
When we convert a poset (X,≤) into a category, we stipulate that
x→y⟺x≤y.
If we now consider the category Set of sets and functions between sets,
and arrow AfB is a function from A to B. If f is
monic, then we know that ∣A∣=∣Im(f)∣≤∣B∣. That is, a monic arrow
behaves a lot like a poset arrow!
Similarly, an epic arrow behaves a lot like the arrow in the inverse poset.
I wonder if quite a lot of category theoretic diagrams are clarified by thinking
of monic and epic directly in terms of controlling sizes.