Astro photos are, by necessity, quite large if only because they document large things. What's more, compared to most photos, it's not the artistic value that counts, but the information density, i.e. how much such a photo can tell you about the object you're studying.
Still, practical considerations keep the actual images, what you get to see published, quite small. Even relatively small images can slow down a browser and eat up a lot of bandwidth.
The European Southern Observatory recently unveiled a huge photo containing no less than 84 million stars, but there is no way to view it all in a browser, ESO built a special tool which enables you to zoom in on the parts you want while keeping the actual image that you see small.
The US National Optical Astronomy Observatory decided to unload one huge file on us though, a 600 million pixel, 600 megapixel photo of the Cygnus Loop, a supernova remnant in the Cygnus constellation.
The supernova explosion dates to between 5,000 and 10,000 years ago and is about 1,500 light years away. The full, lossless TIFF image is 1.65 GB, the lossy JPEG is just 921.23 MB. The photo is 24,457 x 24,457 pixels and covers an area of the sky 45 times larger than the full moon.