Only if you assume it went the seemingly straightforward way, but it could have been more complex. Maybe at first there was no particular limit to unicellular l

Only if you assume it went the seemingly straightforward way, but it could have been more complex. Maybe at first there was no particular limit to unicellular life, and there were unicellular lifeforms both small or large. But the big ones had a terrible problem avoiding getting parasitized by microscopic ones. As, there is one wall to breach, and once it gets in, it's in for good. So maybe eventually one of the bigger ones developed multicellularity as a kind of internal defence wall system, rather than multicellular life evolving from tiny cells clumping together. This gave it an enormous advantage at larger sizes, as all pathogens had to invade the cells one by one, and most of the macroscopic unicellular organisms perished, and some unicellular eukaryotes evolved since then.