Sometimes this can be difficult to wrap your head around in Astronomy, as telescopes generally have a fixed aperture and focal distance, and simply use an eyepiece at the end to make a difference.
If you, instead, look at a camera you can get the concept quite quickly. DSLR cameras have swappable lenses and many lenses include non-fixed focal distances (zoom lenses). So as you zoom with the lens, you are effectively changing the focal distance of the camera, which results in an image zooming in and out. If you've been in photography for a while, you probably also realize that when you do this, it also effects your lighting as the greater your focal distance, the less total light you are actually utilizing. Here's a pretty rough (and simplified) diagram of what is happening:
The crop size is defined by the sensor size. Any light that spreads beyond the sensor is not captured. By increasing the area of the image, the portion that is within the sensor decreases, resulting in a cropped image that appears to be zoomed in.
A telescope works in a very similar fashion, but the telescope has a fixed focal length and fixed aperture. As these are all fixed, the telescope is generally engineered to utilize all light, where as a DSLR level camera will be built to allow diverse lens selection, which results in a lot of light wasted (light that is cropped by not reaching the viewfinder or sensor). The light is finally adjusted by the eyepiece. As everything before the eyepiece is more or less fixed, the type of eyepiece is a direct trade between total light and image size.
It should probably also be noted, that 'More distance = More zoom' isn't true everywhere in optics. Microscopes, for instance, tend to be designed in the opposite fashion.
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