The bottom of the stem will no doubt be at least as thick as the top portion, like stated above, move the light closer to help with stretching, i havent been growing long, but with a 1000w mh over my plants there is absolutely no probs with stretching, the more light the better, just make sure that the heat from the light is not to hot for the tops of the plants, use the back of your hand over the top of the plant to check the heat and intensity. Also, since day one ive had a fan on my plants, they have well developed and thick main stalks as a result, this does help i can assure you. g/l with your grow my friend, keep us posted.