After Draxzel had told the group about some of the dangers that they would find along the way, Click also chimed in about the reason for their size. He explained that they had to evolve to massive sizes for survival purposes. Since the planet was so vast, the various fish, plants, and creatures had plenty of space to grow.
Most things on the Aquarian side of the planet were still relatively small, comparatively speaking. Only the creatures that could withstand immense pressure, or climb the mountain range could get to the kingdoms. The river that flowed out from Oceanus' hidey-hole protected the only dip in the mountains.
If not for that, many creatures would seek out food on the other side...
"To make it over the mountains, everyone will have to ride on my back, or it'll take days to cross," Levi stated as Click finished his explanation.