Where did giant Jack even come from?

Shit , i never thought about that ... hahah

I wondered that too... Because there's no way that it could be produced there. As Skypia can support plant life, but not create it. So, my guess is hundreds of years lalu (Way before Norland) someone went to Skypia and brought a plant with them and planted it there. Then, with the atmospheric conditions in the sky, it grew giant sized. But, that's only a theory. I don't know for sure.