Maybe they are putting up some corrugated tin over the tent outside the gates that was the "club" if you bought a single game ticket?
I don't regularly notice, but do they sell out the Diamond Club seats? must be hard to tell with folks up in the Club itself, but maybe the stub hub experts know.
The Diamond Club attendance tends to pretty much mirror the rest of the stadium but I'd estimate that about 15-20% of the folks will just stay in the club for the whole game. There's a section of tables on the patio that have a great view of the game (my uncle is an amputee who uses a scooter, and when I take him to a game, we stay on the patio tables and don't bother trying to get to our seats) as well as the indoor tables closest to the window. These are usually fairly full throughout the game, fuller if it's super hot or spitting rain, emptier if it's really nice out or a super exciting game. There is always a % of people who don't care about the game and just want to hang in the AC near the bar and drink free beer, too.
As far as actually selling out, for the 2016 season there were no available seats to buy in sec 126 and only 2 pair (not adjacent) for sec 125. That was back in November. Not sure about the other sections.
In 2015 season, I was noticing a lot fewer "Diamond Lounge" (aka Diamond Tent) wristbands than in earlier seasons, which means they are selling fewer of the single-game tickets, at least in 125 where I sit; but, this could be due to Home Plate E having been absorbed back into Lexus Club. The former Home Plate E season ticket holders were given the option of upgrading to Lexus seats or downgrading to Diamond Club, and it seems like most went to Diamond Club seats (I used to sit in HPE a lot and it was a whole lot of regulars). So if the DC section got smaller and the number of season ticket folks didn't get much smaller, then the inventory to sell for singles is smaller.
I'll ask my rep if she knows what is being expanded, and report back if I get an answer!! (updated: got her Out of Office message so it will be January before i have an answer to share).