I usually give a sprinkle of Blood, Fish and Bone in early spring when growth starts, 3 or 4 weeks ago this year. This is a slow release fertiliser that keeps garlic going until harvest. Garlic appreciates a bit of fertiliser, but is not a greedy feeder like squash/courgette or cabbages etc.
I think more important than fertiliser (if the ground is in reasonable health) is to make sure that there is enough moisture when the cloves are getting bigger in June. Often at this time it is really getting dry. Moisture does make a big difference to the harvested weight. Not that small cloves are a problem and they still store well too, but they are more fiddly in the kitchen