That looks like a great harvest!

You should definitely plant them farther apart next time. About 4 inches apart is the ideal spacing to get the most of your space, but still get larger onions. Keep the bed as free of weeds as you can, too. Onions are not very good at competing with other plants for nutrients.

Make sure the soil has plenty of nitrogen. If you're doing organic, that means lots of compost and maybe a little blood meal or other nitrogen supplement. The goal is to get as many leaves on the onion plant as possible before it starts to bulb out. More leaves means bigger onions.

Onions begin to bulb when the day reaches a certain number of hours of daylight. The actual number varies with each variety of onion, (some are 10 hours, some are 14, etc) but that's the life cycle. So to get larger onions, it's best to plant them as early as possible in the year. As soon as you're nearing the end of your frost season, get them planted. That gives them more time to grow before the days get long enough to trigger them into bulbing.