Is my plant eating it's soil?

Ok I have a few plants that are growing quite well but one is growing way faster than the rest. It is only a day or two older than the rest but has them beat by almost 2 inches and is way more lush and full. I am growing them all in the same soil and in 20 oz gatorade bottles that I put tape around to keep the light out and drilled holes in the bottom for drainage. I can still see threw the bottom though and the one with the largest plant is just caked with roots already. The plant is less than 3 weeks old and I have been watering every other day. Today I went to check on them as I always do and I picked up the largest one's bottle to look at the bottom and I noticed the whole thing, plant and all, is lite as hell. It almot felt like an empty bottle. So obviously the plant is becoming a water glutton and drank it all in 24 hours, but why is my soil getting so lite? Is the plant literally eating it? I mean the bottle of soil weighs less now than the day I palnted the seed.

Also I want to clone this thing so I can flower the clone an see if it is a female. However this is my first time trying to clone and I took a small clipping and placed it in soil but the stem I had to plant was like maybe a 1/4 inch long and was not easy to stick in the soil with it still having leaves on it. So the leaves are basically laying on the soil. Is that goin to do any good or am I wasting my time? I don't really know a good spot to clip a clone from. This one I took from a branch that had grown from the joint between the first serated leaf that sprouted and the main stock. It was growing several leafs off of it and new ones were just starting so it looked like a good spot but the stem was still very short as the plant does not have allot of room to grow out, mianly just up. Also anyone know if this plant will make it through a flowering with it already having spread its roots all over the bottle? The thing is about 6 inches tall at this point

Most likely, the soil was just moist when you planted the seed and thats why it was more heaiver then before. The plant now is using up all the water and sucking out all the moistier out of the soil so its lighter.