Their main ingredients have changed to corn flour and rice flour over the years so the cereal may actually be the same size, even though it “tastes” smaller.
There is a trend in American food that has come to be known as ‘volume inflation,’ or when food manufactures increase both the size and height of products without increasing their weight. This results in less calories per ounce than originally advertised but does not necessarily constitute a change in quantity. With cereals, this often involves adding air bubbles such as this advertisement for Honey Nut Cheerios (https://news-media2.wisc.edu/archive/20160524121606/) claiming how “thicker” they’ve become by drawing attention to their height.