The white and red onions have both behaved in the same way and it is the same wherever I have planted them. Unfortunately, I have not been in the habit of keeping a note of the varieties so I don't know if that has been a contributory factor. I shall try harder this year! It may, though, have more to do with the weather we have had over the winter: relatively mild, torrential rain followed by dry spells followed by more torrential rain, and several stormy winds. I've compared notes with a couple of friends who live in the Reading area and they say that their onions have behaved exactly in the same way, which suggest it's down to the weather pattern.
It isn't really a problem as onions are never a major crop for me and I often prefer to use the smaller ones and their leaves when preparing food. But it is nice to have a few larger bulbs when making stews and vegetable casseroles.