For the following answer: when referring to a file that contains the contents of a disk image file we will use "IMG". When referring to a file that contains a picture in an actual visual image format, we will use "image".
If the information in your question is taken at face value
It appears to be mainly due to the limited resolution of the original image file contained in the IMG file, or to the resolution your application used when it converted the IMG file to an image file, be it .png or .jpeg. 4.5MB is not very large for a raw image file. It is quite possible the image file contained in the IMG file was already in a compressed format such as jpeg.
IMG is not a visual image file format per se. It is a format for creating an IMAGE (or copy) of a digital storage drive.
If your question is really about converting files downloaded from NASA's Planetary Data System
The 4.5MB file you downloaded appears to have been the "IMG2PNG" application used to convert image files from NASA's Planetary Data Systems network to .png files. That is the size of the file of the convertor written by Bjorn Jonsson and popularly available on the internet.
If the image file you downloaded was also 4.5MB, then it was not a very high resolution file to begin with. It is likely that the .jpeg version released by NASA was minimally compressed and contained just as much visual information that can be perceived by human vision as the original file you downloaded from PDS.
Also note that when used in the context of imagery from a NASA probe sent to investigate other bodies in the Solar System, referring to a "raw" image does not mean the same thing as when we refer to a "raw" image from a typical digital camera. It just means that NASA has released an image to the public without it first being highly processed and scientifically calibrated to correct for things such as optical distortion of the camera system on the probe, color variations, etc. Most "raw" images released to the public by NASA that are of pictures produced by planetary probes are released in the .jpeg format. Please see this link for more.