Well... that just another law of reasonable physics this game breaks ;p
But of course it is so elementary, you have replied to your own question and did not notice it. It is oh-so-obvious that they can do it only for very short periods of time BECAUSE the super-combine-guy is much larger than the sum of 6 V's. Maintaining the combine state requires lots of energy and thus it is used only on very rare and extreme occasions. That's just so elementary, kids learn such things at physics classes in school! =) ;) :D ;)
But of course it is so elementary, you have replied to your own question and did not notice it. It is oh-so-obvious that they can do it only for very short periods of time BECAUSE the super-combine-guy is much larger than the sum of 6 V's. Maintaining the combine state requires lots of energy and thus it is used only on very rare and extreme occasions. That's just so elementary, kids learn such things at physics classes in school! =) ;) :D ;)