[Formendacil]

On the contrary, other than maaaaaybe around the time of the Last Alliance, I've always imagined Gondor as the larger force. Some of this is no doubt that it is the Byzantium to Arnor's Rome. Some of it is that Arnor was founded by four ships, Gondor by five.

Obviously, going off the ships is probably not indicative of the relative size of the realms, which mut have been substantially based, population-wise, off the pre-existing colonies, but that's actually the strongest argument I have: Gondor has Pelargir, which is said to be the greatest haven of the Faithful, whereas Arnor has, at most, Tharbad.

And looking at the population centres of the realms, Gondor has four clear cities: Osgiliath, Minas Anor, Minas Ithil, and Pelargir. Arnor has two: Annúminas and Fornost (and you could convince me that Fornost was rather minor until the fall of Annúminas).

Thus, although I admit it's more inference than statement, I would say that Gondor was always slightly larger (more populous = more militarily powerful), even if Arnor was preeminent as the seat of Elendil.

Then, beyond that, R.R.J.'s son asked about them "at their peak," and this is clearly Gondor, which waxed ca. Atanatar Alcarin's reign to almost the glory of Númenor (we're told as much in Appendix A, and ruled from Minihiriath to Umbar to the Sea of Rhûn. And part of the reason for that is Gondor's dominant navy, something Arnor is barely suggested as even having.