If I could face consequences for using drugs, I will deny it even after being positive in a test. Of course, once legalized, I'll have no problem saying that I used in once or twice a year. Being it legal, safer and out of the dangerous black market, there will be some new users.
Same happened after alcohol prohibition: more people consumed after the ban was lifted, but consumption was safer. But rarely people that didn't consume during the prohibition went on alcohol binge after the end of the ban. They just drank a couple of beers per week, maybe even a glass of bourbon twice a year, now that they can buy and consume it safely.
Thus the stats you linked doesn't necesarily show a "massive" increase in use, but many people using it sparsely now and many people now admiting to use it that were using previously. In fact, while statista.com shows a 100% increase, the second and more controlled study shows only a ~20% increase that makes more sense (far from massive).
Same happened after alcohol prohibition: more people consumed after the ban was lifted, but consumption was safer. But rarely people that didn't consume during the prohibition went on alcohol binge after the end of the ban. They just drank a couple of beers per week, maybe even a glass of bourbon twice a year, now that they can buy and consume it safely.
Thus the stats you linked doesn't necesarily show a "massive" increase in use, but many people using it sparsely now and many people now admiting to use it that were using previously. In fact, while statista.com shows a 100% increase, the second and more controlled study shows only a ~20% increase that makes more sense (far from massive).