I would say that it is some what strain dependant, sativa's are usually more fluffy then indica's, but I have always found that density is a result of internode distance, the space inbetween each node. I improve my internode ratios by giving the plants more intense red/orange light, like from a HPS.
Also like snuggles said watch your heat but also humidity. IMHO fluffy buds are from not having enough light, in most situations.