The eagle in the manga doesn't look that big to me, and I live in a place where there are a lot of White-tailed sea eagles and Golden eagles, I see them often and sometimes close. Probably a male bird.
Even large female Harpy eagles aren't any bigger than Steller's sea eagles though, both species can get up to 9-9.5 kg in weight, but the Harpy eagle have a shorter wingspan as it hunts in forests. Steller's sea eagle lives in Northeast Asia, mostly in an area around Kamchatka stretching down the Kuril islands to Hokkaido, where lots of them spend the winter.
I guess only the Ainu were in Hokkaido at the time of this manga, but it's normal for these eagles to stray down along the length of Japan, sometimes all the way to Taiwan. So the Japanese should know about giant eagles, and with their huge beaks the Stellers look scary enough. https://en.wikipedia.org/wiki/Steller%27s_sea_eagle