I would suggest the fact that potting soil in pots warms much more quickly and higher than that of the ground and would contribute to faster growth for basil. Also, if you used a slow release fertilizer built into your potting soil your previously potted basil was also being fertilized more. The annual flowers I potted up in Miracle Grow potting soil are almost twice the size as the annuals (same varieties from the same seed I started in my basement) that I planted in ground. I think your ground planted basil will catch up. Give it some balanced fertilizer once in while (I would suggest once a week or every two weeks) and keep an eye out for over watering (yellowing leaves, etc.).

HTH!

Transplant shock can slow any growth, and they should get at least a month of getting used to the new surroundings. Be sure to keep the soil moist, but not soaking wet during the first month. Hope they are in direct sun too, as basil loves sun. Fertilizers are fine in very low amounts. I do not like chemcial fertilizers like Miracle Grow as the herbs have an odd bitter taste which come from the harsh chemical fertilizers. If I were to fertilize the herbs its with very little fertilizer as most do better without any fertilizers as they need to get more flavor and essentials from the pant system itself.