As the percentage of the consumer's income accounted for by a particular good decreases, demand for the good will:

So, let's look at the situation.

Let's say you earn 1000 Dollars. You typically spent 5% of it on pizza - 50 dollars.

but now this percentage will decrease - you will only spend 4 % - 40 dollars. So your budget for this food will be less - you will tend to buy less of this food. So the demand will also decrease.