[Inziladun]

This ought probably to be moved to The Books.

I don't recall any numerical comparison between the two kingdoms. However, I think Arnor was probably the stronger at the beginning.

For one thing, Arnor was the home of Elendil, and I would think a majority of the escapees from Númenor would have wanted to be with him.
Also, Arnor was far from Mordor, and it would make sense for it to be the main training and marshaling area of the army.

It would seem that Arnor took the brunt of casualties in the Last Alliance, and that appears to have set it behind Gondor from that point. The ruinous civil war between Arthedain, Rhudaur, and Cardolan then weakened the divided kingdom enough to allow the Witch-king to finish it off.