There could be different reasons. There are different settings in COncrete5 responsible for automatically resizing images. That's probably one of them.
The first thing you should check is what is the URL to the image you find crisp and what is the URL for the one you find less pleasing.

If the URL only contains folders with names made of numbers and at the end the name of the file, that's your original file.

If the URL is like above but the last folder before the filename is not made of numbers only but represents a name (with or without underscores) then it's a resized version of your image. It was resized using a thumbnail type.

If the URL to the image contains application\files\cache and the file's name itself is a random code of letters and numbers than it's also a resized version of your file, resized using the legacy thumbnailer.

It all depends on how you add the file to the page, which block you're using, how is your theme configures to handle images...