Small sunflower heads
 

I bought some Giant Russian sunflower seeds and planted them out a couple of months ago. Spaced at 18 inches each. Very tall plants but tiny flower heads. Only about 2 to 3 inches across not including the petals. Was hoping to feed the birds with the seeds but looks like there will so little of them. Gave the plants some miracle grow on a couple of occasions.

What could be the problem please?