Several galaxies have angular diameters when viewed from Earth of more than 1 degree. The Large Magellanic Cloud is 11 degrees! None of the planets in our solar system are anywhere near this big in the sky - the Moon is only half a degree.
Getting a brighter picture of something that's dim just requires collecting more light - essentially taking a longer exposure. But getting a sharper picture of something that's very small in the sky is limited by both the aperture of the telescope (the diffraction limit) and the atmospheric distortion, if you're viewing from the ground.
Getting a brighter picture of something that's dim just requires collecting more light - essentially taking a longer exposure. But getting a sharper picture of something that's very small in the sky is limited by both the aperture of the telescope (the diffraction limit) and the atmospheric distortion, if you're viewing from the ground.