It's easy to get these two mixed up. Overtourism and Unbalanced Tourism are two very different perspectives on the same basic problem. One is an snapshot of the current congestion and the other is more of a bird's eye, managerial approach to diagnosing the larger problem from a citywide standpoint and solving it. If an airport is being enlarged to accommodate 100 million annual visitors, but hotels and short-term rentals can only handle 20 million and the top three attractions can only handle 4 million a year, then these things are out of balance. If you're standing in a long line waiting to get into one of those attractions in peak season, you could call that situation overtourism. If you're the mayor of that city, it would be a good idea to ask what could be done to bring things into balance.
When a theme park wants to grow, they don't just add an extra ride. They also make sure they add more parking, toilets, restaurants, cleaners, etc. to accommodate the growth. Without that kind of thinking, unbalance will occur and pockets of "overtourism" will pop up around the city.