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.