Not a radio astronomer (hoping one chimes in), but my understanding is that it’s about sensitivity (what’s the faintest signal it can detect) and resolution (distinguishing signals from points close together in the sky). The Very Large Array has “the resolution of an antenna 36 km (22 miles) across, with the equivalent sensitivity of a dish 130 meters (422 feet) in diameter”[0], i.e. amazing resolution but sensitivity much smaller than the current largest single dish, which is China’s FAST (500 meters in diameter).

[0] http://www.vla.nrao.edu/