Just to add also regarding dev hours and dev team size, it's all relative. If we rewind several years ago, originally Blizzard North starting working on D3. They had a new engine they were working on, but Blizzard or Activision executives didn't like the feel or tone of the engine and visuals, or so the story goes. They thought it was too dark, and then the famous exodus occured of many Blizzard North people leaving the company. Then, a new Blizzard team was put in charge of development, the old engine was reportedly scrapped, and a new engine was developed, supposedly based on WoW, from what I have heard out there.
So really, D3 development was very long and drawn on. TWO brand new engines were worked on, one was scrapped during development. In comparison, for TL2 Runic did not need to make a new engine. Even for TL1, they took the code of an open-source engine and modified it for use in TL1. For TL2 they just improved upon the TL1 engine. Runic has spent over twice the amount working on TL2 as they did on TL1, and that's without spending a lot of time in developing/modifying the engine as they did in TL1. Also the Runic team has grown larger since TL1, so more people working on TL2.
Finally with regards to dev team size, bigger is not always better. With a bigger team, you start to encounter problems smaller dev teams do no face, such as more indecisiveness and diverging opinions, the danger of more code inconsistency and more bloated code, and the danger of a less focused and less disciplined development approach. Feature creep becomes harder to stop. From what we have seen with D3 development, it's quite obvious feature creep was a big problem for D3 development.