I'm aware of the errata, I should have mentioned it but the idea seems to have slipped my mind. (For anyone else wondering, it is found e.g.
here).
You bring a good point about the later books revising the material in the earlier ones. All Falar are medium-sized regardless of sub-race, though there's a large difference in height and weight between the largest and smallest of them (Falanar are about 2.2m while Falaris are average at mere 1.5m, and Falanar are nearly twice the weight of Falaris). Based on size alone, it would fit that their attack size were Medium. It would certainly make things simpler.
As for my own thought-process regarding this, I was looking at Creatures & Monsters tables for lions and tigers for comparison (as Falanar and Falaron are evolved from felines like those). By the way, while tiger is the biggest of the cats but Falanar, evolved from lions, are still the largest of the Falar, probably to account for them being the ruling (largest in physical size) class of Falar. According to C&M both lions and tigers have a Large Claw attack (though Lion still has better OB), so my thoughts were to use Large Claw for Falanar and Falaron, while using Medium Claw for Falaris because they are evolved from panthers that are similar in size to leopard or jaguar that are modelled as Large Cats (who have MCl attack according to C&M).