Increasing the size of your cooktop cutout in a granite slab countertop requires the same cutting process used to create the original cutout. As long as you have a template of the larger cooktop, all that's required to create the new opening is a circular saw and a power sander. You can then proceed to place the cut precisely where needed to make room for the larger cooktop, without chipping or marring the surface of the existing countertop.
Place the template for the new cutout onto the granite countertop, overlapping the edges of the existing cutout hole. Lift the edges of the template and place strips of masking tape onto the countertop surface beneath the edges. Trace the perimeter of the template onto the countertop over the masking tape with a marker then remove the template.
Place the base of a circular saw onto one of the lines drawn on the countertop. Attach a vacuum to the hose of the saw and turn the vacuum on to draw away the dust produced by cutting the countertop. Start the saw then push the blade along the line to cut through the granite slab until you meet the adjoining line.
Reposition the saw onto the adjoining line then cut that line as well. Continue around the cutout area until you've cut away the entire outlined space.
Smooth the edges of the cut with a power sander equipped with 100-grit sandpaper to sand down the granite. If the edges of the cutout are curved, use the sander to round the cut angles along the corner of the cutout space until you reach the drawn lines of the template.
Remove the masking tape from the countertop surface then wipe down the countertop with a tack cloth to remove any residue left behind from the cutting process.