WARNING: The following contains spoilers for Season 5 of The Seven Deadly Sins, now streaming on Netflix.
The titular group of characters in The Seven Deadly Sins are a very eclectic group of people. In a team comprised of a Demon, a doll, two humans, a mage, a Fairy and a Giant, it's interesting that the Giant is the one who is the most unlike anyone else in the group. At 30 feet tall, Diane has always wished to be the size of a regular human, as her tremendous stature has been a major contributing factor to her lifelong struggle with feeling isolated.
Diane's massive size is useful on the battlefield and provides her with enormous strength, but it has almost no practical benefits outside of combat. The Sins like to throw raucous drinking parties inside the Boar Hat, which Diane can only watch through a window from outside. Unable to be a part of her friends' fun, she can't share in their downtime either. While the rest of the Sins stay inside the Boar Hat bar atop Hawk Mama as they travel the land, Diane is forced to walk alongside the giant pig.
At the beginning of the series, Diane's crush on Meliodas was what caused most of her dissatisfaction with her size. Though she loved Meliodas for not treating her any differently because of her height, she believed that her size was the only reason he didn't treat her the same way that he treated Elizabeth. Aside from preventing a romantic relationship, Diane also resented her size because she believed that fighting was the only thing she could do for him, being too large to take care of him when he is injured.
The distance that her size creates between herself and other people is one of the biggest, no pun intended, reasons that Diane can't stand to be physically alone. At separate times during her friendships with Meliodas and King, she expressed to both of them that she never wanted them to leave her side. Being in close physical proximity is all she can manage with most people, so the idea of losing the one thing that makes her feel like she isn't alone is deeply upsetting to her.
Diane's dream of being human-sized has come true a few times throughout the series. Prior to the Fight Festival in Vaizel, Diane and Elizabeth were looking for ingredients for dinner in the forest when they were doused with the spores of a Chicken-Matango mushroom. This sentient mushroom's defense mechanism releases a powder that shrinks whatever it touches for half a day, meaning Elizabeth shrunk to roughly the size of a mouse while Diane shrunk to the size of a human.
Her delight at being human-sized for the first time was short-lived after she approached Meliodas, who failed to recognize her. However, she still decided to do everything she's always wanted to, getting Meliodas to hold her and even inviting him to grope her. Even though she loved Meliodas for never treating her differently as a Giant, she was dismayed to learn that he didn't treat her differently when she was human-sized, either.
Though she inevitably returned to her normal size, Merlin was able to create a magical pill that would shrink Diane to the size of a human for seven hours. This allowed Diane to finally join the drinking parties inside the Boar Hat, even cuddling with King on his Chastiefol. Diane would go on to make use of this pill semi-frequently, though as her relationship with King blossomed, she began to use it less often. Seeing that her size doesn't matter to King made her more comfortable with herself as well.