Infantry is 1:3, Transports are 1:1. Everything else is 1:2.
The advantage to bringing in large armies is that you can spread out to additional capture points much, much faster, and thus get your economy going.
If you have a large starting army and your opponent has a small one, you can just go and crush them at the start. If you have about the same size as your opponent, then the only advantage is the faster economy start.
