You have it backwards

All else held equal sustained turns are helped by larger wing area and lower wing loading. The Mirage’s poor sustained turns come from the drag penalties of the pure delta, a lift to drag issue tied to the wing shape, not a problem from the wing area. It’s also why deltas with vortex lift devices have better sustained turns (and also instantaneous turns, the discovery of vortex lift was basically a rare free lunch for the most part), and why deltas were mostly only found in dedicated interceptor designs until the discovery of vortex assisted lift.