Recently, the Sloan Digital Sky Surveys tweeted a pretty amazing diagram: this scaled comparison of major telescopes, past and present.
One obvious takeaway: telescopes have grown a lot — and there are plans for some truly massive ones to be built in the next decade. (Another takeaway: astronomers like to give their telescopes amusingly literal names. For instance, cancelled Overwhelmingly Large Telescope.)
Specifically, this diagram shows the size of the mirrors used to collect light for each telescope. Some use just a single mirror, while others combine the light from many different small ones. There are a bunch of factors that affect the quality of a telescope's observations, but on the whole, the bigger the mirrors, the more light that can be gathered.
If a telescope is in space, it doesn't need to be all that large to see especially far — which is why the Hubble and the planned James Webb Space Telescopes are comparatively small. But telescopes on Earth have to gather light that's filtered through the atmosphere, limiting how far away they can effectively see. As a result, the easiest way to see farther is to get bigger.
That's why two absolutely giant telescopes (the European Extremely Large Telescope and the Thirty Meter Telescope) are under construction. Using them, astronomers hope to spot all sorts of things that are currently too far away for us to see — planets orbiting distant stars, the creation of new solar systems, and the formation of some of the universe's earliest stars and galaxies.