Lore
King Vendrick Defeated the Four Great Ones
He built the kingdom that is now Drangleic upon their souls and has watched over the land since ages long, long ago.
Vendrick Condemned his own Elder Brother to Aldia's Keep
Vendrick and Aldia both sought the truth, but through different means, and their fervor meant the eventual withering of their familial ties. Weaponsmith Ornifex tells us, "Vendrick came close to unlocking the essence of the soul." And, Chancellor Wellager says, "My Lord made magnificent findings on souls… An accomplishment for the ages." We do not know he methods that Vendrick used, but Aldia studied and experimented on giants and dragons before his disappearance.
The Queen, Nashandra, used Vendrick
According to Chancellor Wellager: Vendrick's queen, Nashandra, warned him "of the looming threat across the seas… of the Giants. The King crossed the ocean… And defeated the Giants, with the Queen at his side." As thanks, Vendrick built Drangleic Castle for Nashandra on top of what is supposed to be the First Flame, which she coveted. Wellager explains, "The King commandeered [the Giants'] power… And created the Golems. With the Golems, the king created the castle. To celebrate victory… And to show his love, his gratitude to his Queen…" Vendrick brought a "prize" back with him from the invasion, which prompted the Giants to invade Drangleic in turn to retrieve it, leading to the downfall of the kingdom.
Vendrick went Hollow
By the time the player finds him, Vendrick has gone completely hollow. He still wears the King's Crown, though, and carries the Ruler's Sword. The item description for the sword reads, "The great king shut himself away, and was soon reduced to a mere shell. Just what was it that he yearned to protect?"
Some players (Like Vaati, Dark Souls lore youtuber) seem to agree that the sizes of especially bosses in Dark Souls is not realistic but more of a way to show the player the bosses strength. So Vendrick, Sir Alonne and other boss humans could just be presented as big bad bosses but actually is just regular human size - But that would not make very cool boss battles (<- Even though i disagree with this :p)