A balloon starts out small - the more we fill it, the more it stretches. The more it stretches the more air it can hold.
Same goes for our stomach.
But here's the difference:
Once enlarged - a balloon can't be shaped back into its original size or smaller. Our stomachs CAN.
Your stomach can stretch with filling, and shrink with gradual reduction. Even if you eat a little less food each time, it is registered and your stomach shrinks ever so slighty in accordance.
Eat a little less each day, and over a month see the difference to your portion size.