Pajama set is made with 96% Lenzing Ecovered Viscose & 4% Elastane. The fabric is a soft texture, super breatheable with a lot of stretch. These are comparable to bamboo fabric. 
 
The pajama was designed to be a snug fit. The fabric does providing you a decent stretch (roughly 3-4 inches). These pjs run true to size. Please refer to sizing chart. 

Chest Measurements: (measured underneath the armit)
12-18M: 19 inches
2T: 20  inches
3T: 21 inches
4T: 22 inches
5T: 23 inches
6T: 24 inches
7T: 25 inches
*Size chart is measured only on one side. The measurements above is for the front & back.

(The set will come with a top and pant only)
 
** Please understand that colors may vary as these are mock images.