It’s that Anurognathus ammoni skull, again.
Pterosaur skulls are fairly light, comprised of bones filled with air passages; but less remarked is how, in most pterosaurs, most of the rest of that skull is essentially air, itself. In fact, the most dense part of the skull are the muscles themselves. But knowing how much of the head’s mass is muscle requires some idea of where the muscles go, so above I’ve done a quick, off the cuff assumption of muscle positions and size (“B” above), and it doesn’t look too favorably for these little guys when it comes to how much muscle motivates the jaw. Coincident with this is how far lateral the muscles are arranged: They don’t wrap around the top of the skull, but are set to the side, and thus are almost completely vertical — except for the palatal muscles, which are mostly horizontal and medial (see “C” above, which describes muscle actions; almost all adductors are vertically arranged, and thus suggests almost no retractive component). Almost everything else is taken up with ligaments, eyeballs, a few additional muscles, nerves, and the vascular complex feeding the face, integument, and nasopharyngeal canal, including the narial plexus. The rest is completely air: nasal passage, cranial air sac system (show above at “A” are only the main antorbital airsac and initial palatal sacs).
It seems, based on a cursory evaluation of other pterosaurs, especially some pterodactyloids, that anurognathids may have had a relatively DENSER head, perhaps a result of the giant eyeballs — assuming I’ve reconstructed them the right size. Adding fleshy integument to this, one might get a comparatively heavy head: