If the FS Ball appears, it would most likely appear near the top of the stage, allowing Bowser to obtain it there and become Giga Bowser without much difficulty. To fit into the tunnels, he would simply crawl into the tunnel, and land in the larger chamber down there and stand. It's rather simple if you honestly think about it. I don't see how it is not that obvious to most.
It's not like he'll roar and snarl and demolish the stage. That would be completely useless and ineffective. He'd never be able to hit anybody like that. And for those of you who don't think Giga Bowser will fit into the tunnels, just make him block the exit and breath fire down there. t's not that hard to think people, is it?