Do you mean you're dynameshing? Or decimating? I'm not sure why it would be doing that unless you're using dynamic subdivision as well (press D after dynameshing)
Basically what I would do originally do when I wanted to create a complexed shape in Z brush would be to import multiple meshes and then decimate them together. However ever since Zbrush R7, I found to my displeasure, I would decimate (No matter how high the setting was) Zbrush would decimate it to some crazy high number…