Years ago when I was learning about fabrics from my Mom, she showed me that muslin had varying weight threads in it so had a rougher feel and tended to shrink more. Percale or other fabric has very uniform threads making it smoother.
I have been disappointed of late that you can't buy muslin sheets and longer. I really dislike the micro sheets, no absorbency at all and stacticy too.
I have a quilt that was made in 1937 and the neutral squares are a heavy muslin.
You should buy the weight you need, if backing light weight and for a neutral block a somewhat heavier weight.
Years ago we bought all our fabric from a catalog. Different fabrics were listed by weight, such as 3 ounces, meaning the weight of a yard. Denim was say, 11 ounce for overalls and percale 3 ounce for dresses or children's clothing. Sure wish it was listed by weight now a days, it would be easier to select.
Ethel