I'd like to think the standard "issue" toa designs would be something like a mix between the metru and the inika as the metru were the missing link between the inika they themselves being a polarizing extreme due to the conditions of them becoming toa of course this is only supported by the fact that their type of set was the most comon.

Another idea which is supported by nova orbis is that this simply the toa's "evolution" the mata/nuva were just made to serve as basicaly police, prepared to fight in order to keep peace but not much more. Then as time goes on conflict builds with the makuta, toa come into existance who are taller and outfitted for combat. Then the war is over and Mata-Nui no longer needs them to maintain him and then they go to a new world and live more frivilous lives so they no longer need such heavily mechanized bodies, thus the toa novus being inika builds