When you import images of any format Construct 2 re-saves it as a PNG using a default stock encoder which doesn't do many optimisations, because it's not as important for your project to be small, only the final exported project needs to be as small as possible. Also if you change an image at all in the editor (e.g. flip or mirror or any tools when we add them) then it can't re-use the original image anyway, so it would be very brittle to try and make Construct 2 always use the exact imported file.
Instead everything's run through image recompression on export to squeeze files down to the minimum size. I would be surprised if, on export, you could beat the compression done by Construct 2 - we've paid to license some specialist tools for the process, and it will do things like count the colors and switch to PNG-8 for 256 color images, and then try multiple PNG compression strategies and pick the best one.
However, if you can beat it, you can safely paste over the exported image files.