Why is my plant so small?

I am just wondering why it is so small? It has been growing for about 5-6 weeks and I know from other peoples grows that they should be at least 3 times the size they are now... Any suggestions on what im doing wrong?

What i have been doing

It is in a 3 gallon pot with a mix of FoxFarm soil, perlite, worm castings, and a pre-mix.
I had it under one 150 HPS, but then switched to one fluorescent tube and 2 100 watt daylight fluorescent bulbs with a color temp of 5000k.
I water about every 3 - 4 days.
I have a one gallon jug with a yeast mixture for added co2 in with it.

They are now outside under direct sunlight. Please any help would be greatly appreciated.

depending on your temps, it could be getting too cold.

How long ago did you put them outside?
Also, genetics?
How close were the lights?

They've been through some nute burn, which slowed them down. Those dark greeen lower leaves, are a sure sign of too much nitrogen, as are the burnt tips. If you haven't fed them, it's probably the soil. FFOF is some 'hot' soil, and some strains don't handle it well. Also, you're probably watering too often. That pot is quite large for a small plant, and even though the soil is drying around the edges, it's likely staying wet in the center, where most of the roots are.

Next time, leave out the worm castings. That strain is sensitive to overfert, and the FFOF has plenty already, especially N.

As for fixing it....the new foliage should take over and give the plant a boost, soon. Either you can wait, or you can carefully transplant to a smaller pot, which'll take care of the soil staying too wet, then transplant when the plant gets bigger.

I put them out about 3 hours ago. The lights were maybe about a foot above them. I have some new seeds germinating so once they are done germinating ill plant them into small pots and by the time the new plant are a good size to repot into a bigger pot, if my currant one isn't any bigger, ill lay it to rest and start fresh with the new plants.

I had the same prob with my first grow one plant stayed 2.5 inchs tall for over a month so I said screw it and flowered it. it shot up to 3.5 inchs tall and stopped surprisingly I ended up with 24.8 grams

your plant is probably mad at you for changing its lighting so much.. stick it back under the 150 if you can..