That's because the greater the magnification, the closer-up things look, but the less the thing you're looking at fits into your binoculars' field of vision. For birders, binoculars with seven or eight power are about right. If you're a practiced spotter and your hands don't shake, you might prefer ten to fifteen power. The narrow field of vision problem also prevents higher-powered telescopes from being good for typical bird watching. By the time you finally locate the spot you're looking for the bird usually is gone.
Telescopes are good, however, in the special case of looking at seabirds and ducks far from shore.
The number on the right of the binoculars' x tells how large the big lenses, or objective lenses, are. The larger the number, the wider the lenses, thus the more light that enters into the binoculars, and the brighter the images appear. Of course the larger the lenses, the heavier the binoculars are, too. If you're big and strong, and don't mind extra weight, you might choose a pair with large, heavy objective lenses, so that colors show up better. If you're well practiced with binoculars and able to immediately spot what you want to see, small, light-weight binoculars with narrow fields of vision might be perfect.