[Agema]

Firstly, just because something is bigger doesn't necessarily mean it needs to be a size bigger. A mammoth may be bigger than an elephant, but what would make a size difference is whether the mammoth has the bulk to knock an elephant out of the way. I'd suggest possibly not - a 6 foot tall man cannot reliably just knock a 5 foot tall man out of his way.

Secondly, why shouldn't Elephants be bigger than Niefel Giants? I could suggest even if a Niefel Giant were taller, it still might not have even close to the same mass as they're much thinner. Generally, it's pretty pointless second-guessing the size of many mythological creatures. Although the Egyptians made a really big sphinx statue, the sphinx was a winged lion with a human head, from which we could assume a sphinx was, well, about lion-sized. Just because there's a 100+ foot-high statue of Jesus in Rio it doesn't mean we think Jesus was 100 feet tall. Similarly, dragons vary enormously in myths. Maybe in Dominions they really are about the size of an Elephant or mammoth.