The characters in many anime series are very stick figureish, I know it is a stylistic decision, but I do not like it. In this case Toriyama’s characters are very realistic as far as body shape is concerned. The muscles are well-defined, and the proportions are realistic compared to many other anime series. Albeit both are exaggerated; with larger muscles than any person you would encounter, and the body proportions, such as the width of the shoulders, is commonly three heads, in this case they are closer to three and a half to four heads wide; we still have the underlying structure of the figures. If this structure is correct, and is understood everything else will fall into place. The characters can be stylized by the artist and still look correct, and believable.
If we look closer at the series from beginning to end you will notice a change in the characters. In the beginning they are still quite muscular, but their muscles are not very well defined. Then as the series progresses you see an increase in the definition of their muscles after each great battle as they increase in power.