Red Onion Sets- all top, no bottom.

I planted a batch of red onion sets around five weeks ago, got them from wilco's. Of around x60 sets I was happy to see that almost all have taken and grown 'leaves' out the top ranging from 2~6 inches in length.

Probem is, I had a closer look at the 'bulbs' at the weekend and noticed that none of them had grown at all, in fact, I poked a couple of them and found they were almost hollow, they look to have gotton smaller 

Is this normal? Will my onion sets suddenly balloon and become big, juicy onions later on?

Confused.

Ah, good, that's what I wanted to hear.

Still learning, thanks!