What though if the dead parts are used as building blocks, if the cause was some sort of virus or bacterial infection that gained control of the victims bodies, the whats to say it can't invade dead cells or cause living cells to grow binding them together. The infection could in fact begin with a newly born child and from there cause it to grow horrifically with the nutrients available. Science fiction I know and probably not ever possible but this is a work of fiction, not a documentary. The behemoths proved this.
The virus (assuming thats what it is) changes the victims DNA controlling growth and other factors creating the creatures. It could be that the different type are from people with unique genetic or phenotypic properties e.g. Growth disorder, high IQs (Burts theory) or lots of muscles. It could also be that the virus is becoming speciated (splitting into different species). This would explain why some types seem to die out and some have just started to appear. Just a thought.