There are many things that can lead to the “fat” shot, but the first two I look for are ball position and balance.
Regarding ball position, too far forward, or back, can cause a variety of compensation moves that, if not properly timed, will cause problems. Also, poor alignment can alter the effective ball position, so never ignore alignment.
As for balance, the swing is like a wheel, the clubhead tracing the outer rim. If the hub–your center of gravity– moves, the whole wheel moves. Swinging with your eyes closed is a great way to correct the problem. Starting with eyes open, take a tiny backswing, perhaps 18 inches, and swing through the ball brushing the grass, finishing in perfect balance. Then repeat, until you can do this with eyes closed, finishing as though you’d made a full swing, facing the target, weight on your target side. Gradually extend the length of the swing when, at each increase, you can successfully “just” brush the grass. This will give you a good sense of better center of gravity control in your swing.