One of my studies for a science course was improving the growth rate and size of plants. The first obvious step was providing the plant with as many nutrients and minerals as possible. I found treating the soil before planting was the best way, mixing in ground bones, tea leaves/coffee grounds, kelp or seaweed and dried fish blood. All seemed to work better than most artificial sources that gave good results the first month, then started to become less effective over time. The slow release of minerals from natural sources combined with them being evenly dispersed in the soil seemed to favour the growth of plants, and encouraged the roots to spread which makes the plant stronger.

If you try this, you should see some good improvements. Although 3.5 pounds is allready large, so you must be doing something right.

If you wanted to get high-tech, you should look at gibberellic acid, the plant hormone that regulates growth. You can buy it commercially at around 90% concentration (which is far too strong for plants). I used it at around 2-3% mixed with water, which showed massive size improvements. Often the treated plant would be twice the size or bigger than the control plant. it was best used as both foliar spray.