While the earpods have been designed to fit a variety of ears, I still find mine a little loose. Being a Sugru nut, here is a step by step on how to add a silicone rubber layer to your earpods to make them fit better on your ears.
Step 1: Getting Started
Once you get your paws on a small pack of Sugru, go ahead to open it. You only need a little per side (roughly 1/5, 1/6 of the pack).
Go ahead and peel some off, knead it a bit, and roll it down into a rod roughly based on the picture here. An EarPod is next to it for estimation.