The phrase refers to content focused on interactions with unrealistic size differences, typically in an SFW fashion. As the name implies, these interactions usually happen with a giant character, and a tiny character, though ordinary human-sized characters are generally accepted in the mix.
"Isn't gianttiny content fetishy?"
"Nah bruh, you're probably thinking of macrophilia or giantess. Most gianttiny content is meant to be cute, scary or funny."
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.