That's the actual classification for them in game. The games only list Small and Large for monsters. Anything beyond that is just personal opinion. The whole Size page is basically full of misinformation and personal ideals, which was something I was going to do about later.
I'm really hoping they change the classification in the series soon. I don't expect any such change to occur in MHX, but possibly in the future afterwards.
Concerning the size page... I'm honestly not sure if we even need it. It's more or less just a bunch of lists of differing sizes, most of which are debatable, and as you said, inaacurate. I could see about maybe revamping it, but I'm not even sure where to begin.
Considering we already have the sizes listed in each monster's page... kinda seems pointless to me right now. At best, the page only seems valuable if it lists a comparison of crown sizes for each monster, but I question whether or not anyone would find that useful for... anything.
Well, my only ideas at this time for revamping the size page would be to add a small summary about how it [size] affects gameplay, and perhaps list "official" crown sizes, along with "event" crown sizes in their own separate listing.