Not enough water... difficult one to answer to be honest. How small do you mean? We never expect ours to get to supermarket size though some usually do. Oh and how did you grow them?

Thanks for this. They got lots of water. I know not to expect supermarket size but most were scarcely bigger than the originals. They were grown in open soil.

Spacing is a big factor. More space=bigger onions. Now seems a bit early to harvest onions anyway. Mine are still growing and won't be ready for another month at least.

Did you put a lot of organic matter e.g. compost in the ground when you planted the onion sets? Also did you keep them well weeded?

Thanks for all your input.
Yes, I measured the space between them and kept them reasonably free of weeds. They were planted in Wickes own compost.
I am now thinking of growing them in containers. I do this with potatoes and they are weed free.

If isn't the weeds it is the nutritional value of the soil not sure that GC compost has enough nutrition in it... Had the onions actually bent over at the neck or did you just decide it was time to lift them? I'd have left them in the ground and continued to water for another month, possibly more depending on where you live in the UK.

Onions are pretty hungry for nitrogen and potash, and need plenty of water, and a deal of space to produce large bulbs. The key is to produce a large shock of leaves before they bulb up--short-day and intermediate day varieties sometimes try to bulb up too early to get good-sized bulbs.

Thanks everyone. I will certainly take your advice on next time around.