Lets say for EDGE we have got a different coding scheme which enables more bitrate giving us more throughput. The analogy in case of E1 will be routers (designed by Cisco) which actually compress/decompress the data coming in/out of the transmission. And hence they may reduce the usage of E1 to half or so.
We have got quite some of these routers in our network and they are doing a good job. 🙂