Yes fertilizer has a lot to do with flower size. Generous amounts of nitrogen (the fish ingredient in the firtilizer) has not that much effect on production of buds and flower size, that has more to do with kalium and phosphorous. However all three NPK factors work together in all parts of the plant's processes. It also has a lot to do with pruning, time of year, temperatures, water/drought and similar. The first flush tend to give larger flowers where I live regardless of optimal fertilizing. I have better results with organic firtilzers than with the other alternatives. However it is easier to get optimal flowers and growth with a combination of organic and chemical fertilzers. Hard pruning is the single most effective way to get larger flowers, but fewer are produced by the plant.