Garment fit is an important part of its performance.
A garment that is too large can create folds that may trap molten metal or burning debris in the garment and cause injury. Garments that are too small may expose parts of the body and will have a smaller air gap between the garment and the wearer allowing heat to transfer more quickly.
The air gap between a wearer and his garment adds to the heat barrier to protect them, this is why multiple layers of thin fabrics can provide better protection and comfort than one single heavy layer.
Wearers must not wear synthetic non FR clothing under their PPE. This clothing can melt and cause burns even though an FR garment is worn over the top. 100% cotton, if it is fully covered by the outer layer, or another FR fabric should be worn underneath the PPE.
On the same token, any coat worn over FR PPE must also be flame retardant. Non FR coats will continue to burn and could actually cause the FR PPE underneath to fall apart and cause burns to the wearer.