In the background text Flexbots are said to be the size of a large dog.
Thus, Neotenics are about same size as Flexbots, but are more difficult to hit, because their small size has been taken into account in the ruletext.
Omission, or something else?
Edit- BTW Dragonflies and Neo-Avians have the same thing going.
Arachnoids are written to have 1 meter shells, but as their legs would protrude so far from it, I won't bitch about that ;)