Tomatoes too small
I'm a novice gardener and this is my first time planting tomatoes ('better boy' variety from costco). There's plenty of fruit, but I'm afraid the tomatoes are too small. The largest tomato I'm getting is about the size of a golf ball. Many ripen much earlier, while they are still the size of a cherry tomato. Is this normal? How can I get bigger fruit? What else should I be doing to get a great harvest? Thank for any tips!
Background: Location is inland southern california. The tomatoes were planted in a raised garden bed over a month ago. I have clay soil, so before planting, I dug a hole about 18 in. below ground level and replaced the soil with a mixture of potting soil + compost + manure, so the bed is almost 2 ft deep with 'good' soil. Fertilized with the osmotote bead type fertilizer. Plants are watered every other day. The plants were already over a foot high when planted, and have grown to twice the size since then. I have 'pruned the suckers' a couple of times and staked the main plants (not a very good job, admittedly).