When most people think of overpopulation, they tend to conflate this with the issue of space, and living comfortably when the population around them is becoming more and more dense. In some places in the world, this unfortunately is the case. Take a look at any drone shot of the streets of Mumbai, where people seemed to be crammed in like sardines, or documentaries on Japanese apartment complexes filled with rooms no bigger than a closet, in some cases, even a cage, due to the amount of available space so limited in those areas.

However, if you look at the global population as a whole, you can in fact fit all of Earth's inhabitants into a space no bigger than the state of Texas, and if piled on top of each other, in terms of just pure mass, it wouldn't even reach the top lip of the Grand Canyon. The real issue when it comes to overpopulation is resources, or "carrying capacity".
Carrying capacity is simply the number of people, other living organisms, or crops that a region can support without environmental degradation. Certain regions of the world have a higher carrying capacity as others, however the real issue lies in the global carrying capacity. As a whole we are increasingly diminishing our resources to the point where they cannot return back to their normal levels. Take a look at over fishing for an example...

This National Geographic article is really interesting and really shows the drastic effects of over fishing
https://www.nationalgeographic.com/environment/2019/02/climate-change-is-shrinking-essential-fisheries/