This is one of the sub-genres of Japanese and Anime. They usually make use of different characters that are overtly sexualized. Some of them would have huge breasts, and there are also instances when they would wear very short skirts.
A lot of the characters would usually be different from how people are supposed to look like. In Japan, hentai is not known to be anime porn, but to the rest of the world, this is how they view hentai. A lot of the videos shared online that are labeled as hentai usually have a lot of pornographic material which explains why people have assumed that hentai is anime porn.