Why Do Plants Grow Slower in Too Small Pots?

Whether olive trees, hibiscus or balcony roses: however different pot and pot plants are, they seem to agree in one: they don't like too small pots. If their roots are squeezed in, they grow slowly and take care of themselves. But why? Does the plant get too little water because its roots cannot grow sufficiently? Or is there a lack of nutrients, because there is hardly any nutritious flower soil left because of root plants? When plants grow in …