That may be a result of measurement. People that used prior to legalization kept it secret. The stereotypical "stoners" are a fraction of cannabis users. After legalization people tend to be more open about their cannabis use.
If the measurement is based on surveys there will be an obvious increase after legalization as the legal consequences of admitting use have been removed.
If the measurement is based on sales there will also be an obvious increase after legalization as the majority of sales are recorded. Prior to legalization the majority of sales were illegal and the only sampling of the actual market size is from police seizures.
Yes, there will be a growth in the market when the legality is changed and stigma is reduced over time. That is people finding cannabis useful for themselves and no fear of being judged for that choice (same as alcohol is for many people).
There will always be a portion of the population that use drugs in excess to the detriment of their health or will compromise their morals to use. There is also a larger portion of the population that uses drugs regardless of legality and participates in society. You would never know the second cohort.
The problem with the first cohort is breaking other laws to satisfy their desire to use. Their drug use isn't the problem. Drugs didn't make them do anything. They should be punished for their other behavior not their consumption habits.