It has nothing to do with the density but specific gravity. Water will measure and weigh the same, FO's will not. It's the chemical compounds used to make up the FO's, each chemical will weight different. If you weigh a FO in a shot glass (with measurments) and 1oz by weight is less than 1oz by volume, then you have a heavy oil. If you order FO's from a supplier that only sells by weight, not volume, and you order 10 different scents that come in transparent bottles, you will see a different fill line on some of the bottles. You may think you are being cheated, but your not, heavy FO's take up less room in the bottle, by weight. That's why you need to measure you FO's by weight for your candles. Some of the known FO's to be heavy are... Vanillas Some of the Spices Sandlewood Patchoulli BTW - With heavy FO's you use less amounts and usually have to use a larger wick (wick-up). HTH Candle Man