I don't think that Blizzard is discriminating against small guilds. They've been moving away from the large guild mentality for some time now, shrinking raid sizes and making content more accessible. I'm not sure what they are seeking to prevent with the 7/10 and 20/25 setting, and I don't think that they see it as set in stone or anything. With time they might relax it, they've shown a tendency to favor solutions that include more people.

They do seem to be moving away from the concept of small vanity guilds or fly-by-night guilds (the ones that invite anyone who is on and make all kinds of plans and then fall apart after two weeks) and trying to encourage people towards larger and more stable guilds. Why? No idea, and I doubt that they'll discuss it. If it's just some kind of experiment, then there's a good chance that they'll relax the rules a bit as time goes on. Blizzard seems to change its mind a whole lot when it comes to WOW.