I would definitely harvest them and enjoy them - and plant more.

Did you plant a different variety of onion this year, Joan? When onions mature is pretty much hardwired to the day length: there are short day, intermediate day, and long day varieties. The size of the bulbs is partly determined by how long they put green growth on before bulbing up, so if spring came early this year, they could have had an effectively longer growing season, in spite of occasional frost later.

Thanks as per, Bathgate. I'll get round to harvesting some of them shortly. Most of them have little pods on top which I am told are seeds and I can keep them.
Thanks also, Tugbrethil. I planted Electric instead of Red Baron this year but can't remember what white onions I planted. I had no idea onions had the qualities you mention. Spring did come early this year, maybe earlier than last so maybe this did have an effect.
Thanks both of you again!