I've found that by using heavy gloves or even very light weights in the hands while striking, and even using resistance bands, builds speed when breaking back down to 4oz gloves. And the repetitive striking builds huge amounts of strength and density to the muscles of the arms. Handstands have also proven to be tremendously effective in mking muscle stronger not huge and ungainly.
When it comes to legs I've found that lifting weights doing squats only seems to make them bigger, and in essence slower because of the added weight and awkward size, not denser or faster. Holding low stances however has helped me to kick roughly three to four times as hard as in the beginning.