Badly worded paragraph. I re-read it many times before I understood.

The "both" refers to the Paramecium and the amoeba. It's saying that despite having all those other organs (and neurons), the wasp is actually smaller than the single-celled amoeba and paramecium.

EDIT: Don't delete your comment. You won't be the only one with that question..it'll help others!

As I understood it, both the Paramecium and the amoeba have a single cell. The wasp has many. (I had to reread that a half-dozen times to figure that out...)

Original: "It’s pictured next to a Paramecium and an amoeba at the same scale. Even though both these creatures are made up of a single cell, the wasp..."

The Paramecium and amoeba, both of which are roughly the same size as the wasp, are single large cells; the wasp is multi-cellular.