Here's why it's considered the smallest:

* Size: It has a diameter of approximately 0.2 micrometers (µm), making it incredibly tiny.

* Genome: It possesses the smallest known genome of any free-living organism, with only 580,000 base pairs.

* Lack of cell wall: Mycoplasmas lack a cell wall, which contributes to their small size.

While other prokaryotes might be smaller in certain dimensions, Mycoplasma genitalium holds the title of the smallest known prokaryotic cell overall.