The lubricants we offer vary in density. The difference is particularly notable when two extremes are compared such as a Polyalphaolefin (PAO) oil and a Perfluoropolyether (PFPE) base oil. As shown in the graphic below, some PFPEs are double density. All the weights of lubricant in the below graphic are the same but you get much less volume of product than you would typically expect with the higher density lubricants.
Oil is sold by volume but an oil is also used as part of a formula to make a grease, and grease is always sold by weight.