It might have been a huge underground basement but when filled with seven other fire based Mecha Dragons, one could say that it's rather cramped in there. When they first arrived at the scene of those dragons, they were actually considered to be on a higher secondary platform. There were elevators that enabled them to descend further down to the basement and the current platform would have allowed them to be at the height of their torsos.
There was a third platform but it can only be accessed at the end of the basement since the Fira Dragon was so huge that the second platform was not able to fully access it. What's more, is that there were even more rooms that connected to the current basement which the secondary platform and third platform was able to connect to.
