You can have wood blocks of different species with the same dimensions but their weight will be different.  Density is mass per unit volume.  In the US, we often report density as pounds per cubic foot.  The difference in weight is largely due to the inherent differences in wood structure of the different species.  These differences are very useful for identifying the genus and species of a piece of mystery wood.  If you were to look under a microscope at a high density species such as oak and a lower density species such as poplar, one immediate difference you would notice is that the oak has very little void space compared to the poplar.  Oak has low porosity and permeability.   Oak will have large pores in the earlywood (springwood) of a growth ring but the rest of the ring is packed very tight.  White oak commonly has a condition known as tyloses.  This occurs when an adjacent cell grows through a pit in the cell wall of the large earlywood vessel and essentially clogs the vessel.  So, white oak has very low permeability and is difficult to impregnate with preservative treatments.  However, white oak is an excellent choice for cooperage – liquid containers.  I have had the pleasure of visiting a wooden barrel factory in Kentucky.  The assembly process remains unchanged throughout the years as does the wood species – white oak.