Radio telescopes pick up radio waves for analysis. They "see" things that are too far away to see in normal light, so they use radio waves. Satellites often use parabolic dishes with radio telescopes. The larger the dish, the easier it is for the satellite to observe weak radiowave sources coming from deep space. A larger dish also allows more detail to be seen by increasing the resolution. Unfortunately, the larger a dish, the more difficult it is to send into space. Engineers must find a balance between the efficiency of the radio telescope's dish, and the cost of sending a large dish into space.
Remember: with each extra pound a satellite weighs, it costs more money for the fuel and for the space it takes up in either the shuttle's payload bay, or on a rocket.