Any corporation that is going to base itself on beta would have to have a minimal amount of firepower, to fight off SO's if nothing else. I think the worry here is that a small corp that captures an outpost, will again choose not to base there, since they would have difficulty moving supplies, mining, and just otherwise controlling the area around the outpost.
When I think of small or medium, I'm more considering the time zone coverage. A 'small' corp would still be an average of 20 active players online (about 60 player base), but they may only cover a 4-6 hour window; where a medium corp could cover 6-12 hours (200 players), and a large corp could cover 24/7 (500+).
It would totally defeat the purpose of the outpost change if it ends up in the same situation again where a corporation can't keep the area safe so they end up mining and ratting on Alpha and simply run back to beta each time the SAP goes active.
An alliance supported scheme, that is allowing multiple corps to band together and secure outposts, is just going to support the current model of play under the new mechanics. The ability for same Island corporations to be Allied isn't stopped by the game and is actually beneficial for both. Requiring each corp to be able to hold their own outpost though ensures that they can be equals in the alliance. It also doesn't stop outpost oweners from making alliances with small corps and allowing them access to both the island and the resources. This is actually better overall, since the outpost owner can't just use the small corps as a meat sheild, so the small corp actually needs to be doing something Useful for the alliance; which can simply be using the outpost which provides the owner with NIC.