Compressed images are altered to have a lower resolution and thus a lower image size. If you want your compressed image to revert back to its original size, the only way to do this is to keep a copy of the original.
Compressed images cannot be increased in size, even if they were originally larger files. Why can’t compressed files be “uncompressed”?
When you use a JPG Size Reducer to compress your image, the image is permanently altered. All non-essential pixels that do not make up the integrity of the image are removed.
This means that the image will still look exactly the same without any noticeable difference, even though it has been drastically reduced in size.
Once the non-essential pixels are removed, they cannot be recovered. Generally, computers are not able to create pixels to increase the image size, and thus small images cannot have their resolution increased.
Each image has a specific point in which all the remaining pixels are vital to sustain the quality of the picture displayed. Removing any more pixels than this is termed lossy compression. In that instance, the image quality is compromised in favor of a significantly reduced size.
If only the non-essential pixels are removed and the image quality is maintained, this is termed lossless compression. In this instance, the image is only compressed to the point where quality is not compromised. Under this type of compression, the image may not be reduced as significantly as would have been the case under lossy compression.