>>40521230the chars have a flame bladder inside their body that produces flammable gasses, the end of the tail actually has an opening for excess gas to escape through and that's what keeps the flame continuously lit
think of it sort of as a pilot light
this is why when they get powered up the tail flames burn larger and brighter because even more gas is being produced and spilling out
so there's the reason charizards have such a big round belly, in addition to all the normal organs there is the whole flame system with the gas bladder and what not. One other advantage though is the lighter than air flammable gasses aid it in flight by reducing its body weight.
interestingly during the mega transformation it seems the flame productions are so rapid even the tail opening isn't enough to get rid of the excess so they start spilling out of the mouth too
