One tip I have about onions and garlic: The bulbs underground are not roots, but specialized leaves. Feed/fertilize them as if they were greens, not roots. They like more nitrogen than you might think. They also do not like competition from weeds, so mulch the beds or keep up with weeding.
Disclaimer: I grow great garlic, but puny onions. Part of that is the type of onion I grow -- multiplier onions, which don't make very big bulbs, but they should be bigger than I'm getting. My best performing onion is Stuttgarter.
Also, make sure you're growing the right type of onion for your latitude. Up here in Michigan, I grow "long day" onions; farther south (Mason-Dixon line-ish) "short day" onions are better at sizing up their bulbs.
It's actually not the length of the day, it's the shortness of the night that triggers the bulbing on the plant.