Friendship is a common theme. Just like in the regular My Little Pony stories, the giantess ponies often form friendships with other ponies, despite their size differences. They learn to accept each other and work together.
One popular 'My Little Pony Giantess story' could be about a giant pony protecting a small village from an outside threat. The pony's large size gives it the power to fend off the danger and become a hero in the eyes of the villagers.
It could be Meg. Since she's often the outcast in the show, turning her into a giantess could be an interesting twist. It would give her a new - and very literal - form of power and attention that she doesn't usually get in the show.
A 'giantess cuckold story' typically involves a fictional scenario where there is a giantess character in a relationship dynamic that may involve elements similar to the concept of cuckoldry in some way. However, it's often a very niche and specific type of fictional or fantastical narrative.
No. Popular stories usually have themes that are more widely understood and appealing. 'Giantess cuckold stories' are extremely niche. They involve rather complex and not - so - common relationship dynamics along with the very specific idea of a giantess. Most people are not drawn to such stories as they are far from the typical story elements that are popular in mainstream literature, movies or other forms of media.
Giantess girl comics are known for their imaginative scenarios. They might feature giantesses interacting with normal-sized people in unexpected ways. The themes can range from adventure to romance, depending on the creator's vision.