Just to clarify I am not saying that the Gigas worshiped Sephirot. More, they are the result of experimentation on Sephirot.

We know for a fact the reason Allagans like Xande and Amon are so mahoosive is because of magical growth juice extracted from Sephirot. So we can assume that the rich and powerful turned themselves into giants make themselves more powerful and impressive. A symbol of their dominance or something. Like ow rich people but expensive cars and such.

And then there's Phlegthorn, another giant Allagan. We know from is triple triad card that he was actually an enemy general that was captured and brainwashed, given the 'make my monster grow' treatment and left defending the labyrinth of the ancients.

So here's my theory...