Complex sentence: Compared to the population density of England and Wales which has stayed the same and slightly decreased around the 1900 but then increased soon after.The density changes a lot from one census to another but that was also down the change of map boundaries.
Split into simpler sentences: Compared to the population density of England and Wales which has stayed the same and slightly decreased around the 1900 but then increased soon after. Although the density changes a lot from one census to another but that was also down the change of map boundaries.