Welcome to the forum
You have to keep in mind fish are constantly releasing ammonia from their gills, which will build up without water changes. Clean water keeps the immune system of a fish strong so they are healthy. Of course larger tanks help dilute the build up of toxins meaning less water changes, which is why larger tanks are typically better.
In a two gallon, you'll have 2 50% water changes and a 100% water change a week. In something like a 5- 10 gallon tank, if you cycle it, you could get away with one 50% water change a week.