If the flowers are the same colour as last year it may be that because roses are greedy feeders and like a lot of fertilzer, good soil and water, if they don't get it the flowers suffer from the lack of vigour in the bush. Also if the roots have been damaged, such as sitting in water too long or not enough water, disease or insects, then that too will make the flowers smaller. If the flowers are not same colour, such as a semi double red, then the understock may have taken over because the top died.