I think people usually start onion seed off earlier than that in the Green House.Sowed some Ailsa Craig myself at end of April. Some grew big a lot small.
What fertilizer did you use?
Kelsae or Robinsons Mammoth are the ones to grow if you want big onions. exhibitors will have already sown, the shortest day is the traditional time, but I sow early February (in a frost free greenhouse) and got big onions. You can prepare the onion bed over the winter. Big onions like a lot of food, and no competition from weeds.