We design our muslin pieces to sit loose on the body, not fitted. Muslin is a cotton woven with an open, loose weave, and that gap between the threads is what lets air move through the fabric. If a piece hugs too tight, you lose some of that airflow, so we cut ours with room through the chest, tummy and legs rather than snug like a stretch fabric. Think relaxed and roomy rather than second skin.
Go by your baby's current height and weight rather than age alone, since babies grow at different rates and muslin doesn't have stretch to make up the difference. Our sizes run roughly newborn through 4 years, so if your baby is between sizes, sizing up gives a bit more wear before they outgrow it, while the loose cut still looks right rather than oversized. Muslin also softens and relaxes a little more with each wash, so a piece that feels slightly stiff or creased out of the bag will drape looser after the first few washes. Creasing is normal for this weave, not a flaw.
Fit holds up better with the right care. Wash cold on a gentle cycle, then tumble dry low or hang to dry, and skip fabric softener and high heat since both can work against the weave over time. Our fabric is double layer muslin, pre-washed before it reaches you so it's already soft and closer to its settled size, and it's sewn with flat seams so nothing digs in at the fit points like underarms or the waist. If you size up or down and it's not right, we take unworn returns within 30 days and ship worldwide with tracked delivery, so getting the fit sorted shouldn't be a hassle.