Normally you cut a pizza into slices from the centre, so each slice is an isosceles triangle.
Suppose I go "YOLO I'm cutting this straight up" and I just make one giant vertical slice straight through the pizza (not necessarily through centre) so I get one giant piece and one little piece.
The little piece is called the minor segment, and the larger piece is called the major segment.
You already know the formula for the area of the minor segment. The area of the major segment is easily found by getting:
Major Segment = Whole Pizza - Minor Segment.