In other words, they contain more regions of DNA that are expressed. The idea that genome density decreases as the complexity of the organism increases hold true. This is due to the fact that eukaryotes have a much stronger presence of introns than prokaryotes.
Essentially, these organisms feature additional sections of DNA that are expressed. It holds true that genome density decreases alongside increased organism complexity. This is because eukaryotes have an abundance of introns, unlike prokaryotes.