TBMs can turn, albeit slowly. Bigger tunnels are not necessarily more efficient than parallel tunnels. Consider changing a 1 lane to a 2 lane tunnel. You can dig two adjacent tunnels, or you can use a TBM that is twice as wide, and therefore has to remove 4x as much material. Even if you can only build 2 or 3 TBMs per bigger TBM, it's a win.

The tunnels are still pretty big, and the sleds could be 10' wide and 10' vertical if the tunnel ID is 14'. That's pretty comparable to a subway or bus, and the increased distance between the sleds is made up for by the increased speed.