How do you make your Spore more girthy?

Hi, I just purchased Spore on my mac and I have just started my new carnivore species. I'm in creature stage and I want to make a massive troll like race. Think is i only seem to be able to adjust posture and length on the torso as well as legs and arms. How would i go about increasing the thickness/girth of my creation?

well use your mouse to highlight a section of the spine and use your mouse wheel to increase size, i use my mouse wheel, im not sure what button to press to increase size tho