I used map.fitbounds(circle.getbounds()) but this worked for particular radius change. But I need to zoom in and out for every radius change.
For example: radius is 200km, when radius reduced - zoom in and when radius increased - zoom out
// ...user changes radius...
var bounds = circle.getbounds();
// Set zoom to max possible
map.setZoom(20); // 19 or 20 or get the maxZoom property of your MapOptions
map.fitBounds(bounds);
map.panToBounds(bounds);