I can't get over the suspicion that all those viral pictures are photoshopped and 'Flemish' belongs in the lower right circle.
This comic is a Venn Diagram (a more exacting subset of Euler Diagrams) grouping different things with "giant" in the name. The three categories are giants from space, 'geological' features called giants and giants who have been proven to be fictional. In the central overlap of all categories is a possible 'giant', fulfilling all groupings, but normally without the word "giant" in its name.
Venn diagrams and Euler diagrams are the subject of numerous XKCDs, including 112: Baring My Heart, 773: University Website, 1180: Virus Venn Diagram 2122: Size Venn Diagram, 2721: Euler Diagrams, 2769: Overlapping Circles, 2903: Earth/Venus Venn Diagram, 2962: President Venn Diagram, and various others.
- Red Giant (Space)
- A type of large, relatively cool star.
- Blue Giant (Space)
- An early type of large, relatively hot star.
- Iron Giant (Space, Not Real)
- Title character from a 1999 animated film and the 1968 children's book it was based on (written by English poet and author Ted Hughes and published outside the US as The Iron Man: A Children's Story in Five Nights) : a robot from outer space.
- Frost Giant (Not Real)
- Beings from Norse mythology, Frost Giants also appear in Marvel comic/film versions.
- Jolly Green Giant (Not Real)
- A mascot for a brand of canned vegetables.
- Cardiff Giant (Geologic/Planetary, Not Real)
- An 1869 archaeological hoax of a supposed petrified giant man.
- Atacama Giant (Geologic/Planetary)
- A prehistoric geoglyph in the Atacama Desert.
- Salt Giant (Geologic/Planetary)
- A huge salt deposit below the Mediterranean sea.
- Gas Giant (Space, Geologic/Planetary)
- A large gaseous planet, like Jupiter or Saturn.
- Ice Giant (Space, Geologic/Planetary)
- A relatively large icy planet, like Uranus or Neptune.
- The Man In The Moon (Space, Geologic/Planetary, Not Real)
- A 'face' visible in the near side of the Moon, sometimes shown as a character in children's nursery rhymes. As a feature, it is real, but not a 'real' giant, only an instance of pareidolia.
- Flemish Giant (title text) (Real, but Randall suspects it belongs in Not Real)
- One of the largest breeds of domestic buns, about the same size as a Cocker Spaniel dog. This means it is a very low-ranking one.