On the subject of chambers, there won't be any specific bonus added for small ones, but there might be reasons you would want to build a small chamber.
- Big chambers will have more upgrade potential as tiles of the same type need to be touching for them to be upgradable (it's a bit more involved than this, but bigger=more upgrades is the general idea).
- If chambers are too big, there might be a chance that the central tiles will collapse (which would be an expensive loss)
- Small chambers allow you to have more customization in your groups. You can only move rooms of ants between groups (you can't split them up). If you make one huge room of ants, you will only be able to do one thing with them at a time. This could become troublesome if you need to defend from small attacks on multiple fronts.