Did you know that a gallon of water and a gallon of glycerin does not weigh the same? This is because glycerin has a higher specific gravity than water. The specific gravity of water is 1.0 and the specific gravity of glycerin is much higher at 1.263. The higher the specific gravity, the heavier the liquid.
Fragrance oils are typically not made with glycerin but are made with a combination of ingredients that produce different specific gravities. Depending on the combination of ingredients the specific gravity of fragrance oil can be heavier (over 1.0) or lighter (under 1.0).
Fragrance oils are packaged in glass bottles because the glass does not interact with the aromatic ingredients. The glass ensures the fragrance's aromatic properties remain as strong as possible, and also glass is able to withstand the potent properties of fragrance oils (unlike plastic). Fragrance oils that are packaged in glass must be sold by fluid ounce because adequate air space is required to prevent the glass bottle from cracking when the oil expands during transport or in warm months. If bottles were filled by weight, heavier oils would not have this air space. The typical fluid fill line for a glass bottle is the bottom of the rounded shoulder of the glass bottle.
The SDS document will define for you the specific gravity of fragrance oil. It is located under section 9 – Physical and Chemical Properties.