Just what I was looking for -- cotton (not satin), simple and inexpensive.  I ordered both the 12 mo and 18 mo (my son is 21 lbs).  I found the two sizes to be similar in width and arm length.  The 18 mo was about 1 inch longer, so I kept the 12 mo and returned the 18 mo.  It is a little sheer, but a onesie will solve that issue.