It would make more sense to compare carbon emissions using each crop's market value rather than its weight. No surprise that growing cannabis takes way more carbon output than growing the equivalent weight of corn, a low value crop which practically grows itself!
It's rather ironic - the whole indoor growing started out only because of the illegality, but now even in the places it's legal, it's still being done that way.
What's more, indoor grown cannabis is being increasingly seen as higher quality product and is sold as high end 'craft' bud. It'll be interesting to see where this goes considering the slow but steady acceptance and legalization in many parts of the world.