Telescopes are designed with the idea of gathering as much light as possible, retaining as much detail as possible, and making the image large enough to be seen well. The idea behind using a telescope is to increase the effective size of our pupils. The amount of light received is related to the square of the diameter of the collecting lens or mirror. If we use a lens or mirror twice the diameter of our pupils we gather four times as much light. This is because it is the least important.
Magnification works against light-gathering power.
It does this by spreading the image over a larger area. If we have a 15cm telescope operating at 100 power the image appears six times brighter than it does to the unaided eye. However, if the same telescope is used at 25 power, the image is 24 times brighter than with the naked eye. It is for this reason that small telescopes do not perform well. While any telescope can be operated at any magnification, no telescope can perform well at more than about 2 power per millimeter of aperture.