I admit that I only seen pictures of the daemon lords so I don’t have a great gasp of the size comparisons.
The normal sized avatar is small when compared to normal sized greater daemons. My normal avatar is easily a head shorter than my Keeper of Secrets; it would be more if the model wasn’t doing its slanted crouching down pose. The forge world model may be small when compared to the other daemon lords, but his model dwarfs the other monstrous creatures in
40K.
I think the model justifies gargantuan status, just my opinion. Plus if size alone was the decider then great knarlocs would be monstrous creature and the C'tan would be infantry.