All your pictures are blurry or focused on something other then the plants (the soil in the background, etc.) so it is very hard to see how they are doing. Try to learn how your camera works so as to be able to focus it better on the actual plants.

I don't think they look too terribly small for their age, but I could be wrong. I don't have much experience in growing... tried to grow last year and got all males, and almost totally wiped out by a severe thunderstorm this year. So take my words with a grain of salt.

The miracle grow normally has extended release nutes in it. Regardless, feeding twice a week would probably be way too much. Here is a good link concerning the nutrients for cannabis. Give it a read. Nutrients And Marijuana Grown In Soil

One thing I noticed is that all your pictures look shaded. It could just be the quality of the pic though. But just in case... how many hours of direct sunlight are they getting a day, in your estimation?