Actually both Tokio Jungle and Rain are C.A.M.P., a team of promising new developers that Sony gives resources to make a game. I guess they then keep the best of them and integrate them.I edited my post. London "only" has over 300 staff.
Yeah, Sony Santa Monica are huge. 2 large development teams, support for other first/second parties, R&D and the two divisions you mentioned.
Japan Studio has a large R&D team, so they take up a significant part of Japan Studio. Then they've got at least 4 teams (Last Guardian, Gravity Rush, Rain and Puppeteer) of varying sizes (none of which are near the size of Sony's AAA western teams) and second party support (Soul Sacrifice, Tokyo Jungle, etc.). I guess they could merge those teams together into larger teams but if people think that's going to result in more games, I'm pretty sure they'd be mistaken.
Btw, since Becker has joined Studio Japan we've got announced or released: Gravity Rush, Tokyo Jungle, Rain, Puppeteer and Soul Sacrifice out of Studio Japan. That's a pretty good turnaround (I'm counting in SS because it is produced by Studio Japan in, what I imagine, the same way SSM helps out other smaller studios).