Why is cell count stated relative to animal size?
The typical goal of scRNA-seq is to assess the possible cell-types and states that exist in an organism/tissue/etc. Given that, it can be useful to know what portion of the possible cells have been assayed, since that sets bounds on the number and size of cell populations not assayed. If the goal of a study is to assess the variability in a single cell-type, then you're correct that mentioning "0.8% of the population was sequenced" isn't actually informative. Note that "in a single cell-type" is important there, since if one is assaying multiple cell-types then it's important that enough of each are assayed given their prevalence in the relevant population.

Practically speaking, numbers like this are only ever written in papers if they look impressive, so no one is going to write about sequencing 0.000001% of the cells in an organism, since the reader/reviewer will then start to question the relevance of the results.