Apologies if an ignorant question, but would a telescope in space require a much smaller mirror to achieve the same quality of an earth based telescope?
Weight allows you to put many more scientific instruments on the same telescope (fiber-fed spectrometers, intregral-field spectrographs, imagers, coronagraphs, etc.) In order to put an instrument on a space telescope, you have to miniaturize it and make it light, and you have to decide before launch what instruments to put on the telescope. In contrast, on the ground, you're not nearly as weight- and size-constrained, and you can upgrade the instruments on a telescope every several years. There are decades-old telescopes still in use, retrofitted with modern sensors.