A paper with higher density has lower bulk, meaning it is more compact. And of course, the opposite is true for a paper with low density. It has more volume and contains more air.
The term bulk is used to describe the volume of an object, and paper bulk is measured in cm3 per gram. If 1 gram of paper has a volume of 1 cm3, it has a bulk value of 1.0. So a low bulk value means that the paper is compact, with little air between the fibres. A paper with high bulk value instead has a lot of air between the fibres, making it thick, but without adding weight.
Let's take a deeper look
Imagine that you have a sheet of paper that weighs 60 gram per square metre. This paper has a bulk of 1.5 and a caliper thickness of 90 µm. If you compare this paper with another paper of the same grammage, 60 gsm, with a bulk of 2.0 the caliper thickness will increase to 120 µm. This means that the paper weighs the same, but the sheet is actually thicker because it has lower density and contains more air.
What does that mean when buying paper then?
Well, you need to stop thinking about your ideal paper as defined by its grammage or weight per m2, and instead focus on the thickness of the paper sheet. If you do that, you will see that a high bulk paper will be able to give you the same thickness in µm, as your old paper, but it will weigh less because it contains less paper fibres and more air.
And why is that a good thing?
You buy paper in tonnes, right? But what you really want to get out of that massive, potentially toe crushing reel is square metres of printing surface. If the paper weighs less per m2, you get more printing surface for each tonne that you buy. In some cases, the difference in weight can be as much as 16%. Imagine how much money that is!