[tom the eldest]

I think it has to do with arnor smaller population.arnor was suffering heavy losses at the war of the last alliance.they were already dwindling when angmar atrack,from migration to the south,harsh winter,and civil war.so,the sucessor state were greatly weakened when angmar attack,although they maybe coud still muster 20.000-30.000,because at the siege of fornost,arthedain had 10.000,remnant of arnor's troops.