So... why do you figure the 10% rule is there? Given that no real-world spider weighs even half a pound. I'd say "normal" means no magical powers or human-like intelligence, not necessarily within real world parameters. A lesser Great Spider is just a big spider. I could certainly see insisting that he find one in the wild, rather than summoning it, but there doesn't seem to be any good reason to forbid it as a familiar on either game balance grounds (using it as a familiar and as an assassin would be a dangerous game) or on world-internal grounds (as far as someone living in a RM world would be concerned, a lesser Great Spider is just a really big spider, though the other Great Spiders are something more). It'd be more durable than an housecat (assuming average specimens), but not quite so durable as a ferret. The venom is really the only significant advantage, and you don't really want your familiar going into combat if you can avoid it, so that doesn't seem like big deal, especially as anyone could develop some animal handling and buy some big war dogs which can be treated as expendable.