Someone on the discord server ? server name? (soul series datamine created by Zullie) confirmed interesting things about the giants in DS3.

First that the yhorm model was not created with a giant in mind, its model comes from a character that was removed due to DS3 being rushed, he was Berserker halleck not a giant but strong warrior.

Originally you had the option to use a bonfire ceremony to create a world state named "past" and then 4 Gwyn knights and giants appeared in anor Londo.

Most of the giants in DS3 seems to be because of DS3 being rushed.

Honestly in my opinion retail DS3 and DS2 feel like two diffrent ways of making the same game.