Massive size isn't really an issue for a summoner class, because summons tend to be powerful astral beings. This means that size isn't set in stone for them, they can adjust it as needed. heck, they can probably alter their appearance as well, and in FFXI the beings you summoned weren't actually the being itself, but it's avatar, a sort of projected image that did it's dirty work for it. Actually calling the summon itself, like the hero of windurst did for fenrir, grants you incredible power that the avatars can't compare to: think level 500 fenrir or something: but in exchange it costs incredible amounts of magical energy by the caster and basically kills them.
If beings in this game are anywhere near similar in power magically, they could probably also create avatars of themselves, and so size could be adjusted to fit the game.