I was about to point out that the Myrmidon is smaller than the Serapis, when I remembered that the Serapis is also much thinner and more agile.
It also has half the shielding.

I'd also say the Hercs are all so chunky as to be nearly round. The Myrm would be perceived as vaguely flat(well, sort of awkwardly arched; there are still the humps on all the weapon 'pods', making it look big for no apparent reason), if not for the cockpit pod and the backwards-facing hook hanging down in the front.
Just look at how much ship there
isn't(in the middle) when viewed from slightly below.
Second the comment on the Dragon. Either they curl into a little ball, or they somehow stretch out their arms into the wings, or some combination thereof.
Lack of Harpoon, while not a dealbreaker, IMO, does hurt its ability to make use of the small missile banks somewhat. It is odd/amusing that it can carry the Harpoon D.
One benefit of the tiny banks that I did forget about, is the relatively quick reload with the Hornet/Tornado. Since all banks fill simultaneously, it would take about half as long to reload the tiny banks with Tornadoes on a Myrm(assuming the large bank has Tempests, Helios, Trebs, Stiletto IIs, or hasn't been emptied yet) as on a Perseus(or other 40-Hornet missile bay).
The main FS2 campaign really doesn't help the Myrm at all, denying you the Tornado(and thus any usable/compatible anti-fighter aspect seekers) and the Prom-S until you're with the 64th Raptors, then you get shipped out to the Psamtik a few missions later, and when you finally get back to the Aquitaine, you've got the nice, shiny, 8-gun Erinyes, that largely outclasses the Myrm in just about every way(and most likely, the sickeningly durable, destruction-spewing Ares), waiting for you.