The weave is the whole story. Muslin cotton is woven loosely and openly, so there's more space between each thread than in a tighter cotton weave. That space is what lets air pass through the fabric instead of getting trapped near your baby's skin. Epic's clothes use two layers of this loose weave sewn together, which sounds like it would cut down on airflow, but the gaps in each layer still let air move. We explain how double-layer muslin cotton works if you want the full picture.
Regular cotton, like the kind used in most baby bodysuits and sheets, is usually woven tighter and finished smoother. That tighter weave holds its shape better but has fewer gaps for air to escape through, so it can feel warmer against the skin on a hot day. Muslin trades some of that structure for airflow, and it's lighter in the hand too. We lay out the differences in more detail in our muslin cotton vs regular cotton comparison, including how each holds up in the wash.
Wash matters as much as weave. Machine wash Epic's muslin cotton cold on a gentle cycle, then tumble dry low or hang it to dry, and skip the fabric softener and high heat. Muslin softens more with every wash, so a piece that feels a bit stiff out of the packaging will feel different after a few weeks of normal use. It also creases easily, which is just how the loose weave behaves, not a flaw. If you're wondering whether that softening holds up for a newborn's skin, we cover it in is muslin cotton soft enough for newborns.