You will need more light as they get bigger. Aim for 5000+ lumens per square foot of growing space. CFLs and fluoros are only effective 1-3 inches from the plants, so you should surround them all with lights to get the best penetration. Otherwise you can look into LST or Scrog to get an even canopy so you can mount all of your lights on a flat surface above the plants. This makes it easier to adjust the lights as they grow taller.

There's a pic of 2 plants in a 51cm x 51cm scrog. Nice, tight, uniform nugs all throughout.