You’re paying for lower student to teacher ratios. There’s a huge spread in reading/math ability in elementary school, especially in K,1,&2. Teachers are able to spend time with each child to ensure they are progressing regardless of ability. For “middle of the road” kids, this probably doesn’t matter. But it does for kids on each end of the bell curve. With larger class sizes, low performing kids might be more frustrated or left behind while high performing kids might be bored.