This is definitely part of it, the estimates claiming 90% bacteria by cell count end up suggesting perhaps a half gallon of actual bacterial volume. As far as DNA, I think the parent stat can't hold for base pairs. Not as a volume issue, but because mammalian genomes are ~3 orders of magnitude larger than bacterial ones. Even at a 10:1 cell count, our base pairs would be 99% human.

Most importantly, though, the 10:1 stat is just outdated. Newer estimates put the number around 1.3:1 instead.

https://journals.plos.org/plosbiology/article?id=10.1371/jou...