The weave tells the whole story. Muslin cotton is woven loosely and openly, leaving more space between each thread than you find in a tighter cotton weave. That open space 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 reduce airflow, yet the gaps in each layer still allow air to circulate. Read how double-layer muslin cotton works if you want the full picture.
Regular cotton, like the fabric used in most baby bodysuits and sheets, is usually woven tighter and finished smoother. That tighter weave holds its shape well but leaves 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 feels lighter in the hand too. We lay out the differences in more detail in our muslin cotton vs regular cotton comparison, including how each fabric holds up in the wash.
Washing 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 further with every wash, so a garment that feels slightly stiff out of the packaging will feel much softer after a few weeks of normal use. It also creases easily, which is simply how the loose weave behaves rather than a flaw. If you are wondering whether that softening holds up for a newborn's skin, we cover it in is muslin cotton soft enough for newborns.