What makes an image heavier than another ? For example an image has 110KB ,while the other has 60KB .What makes the difference ?

Answer 1

The way images are compressed, i.e., the extension difference, and the size of the images.

Answer 2

The size of image is depends on its resolution, bits of color.

Example: if an image is 1000*1000, the image has total 1Million pixels. Each pixel among these 1 million represents n bit color.
Suppose: It has 8 bit color pixels. If the picture is compressed to have only 4 bit color pixels, then the size of image will be half of the original.
The compressed image also looks similar to the original, but actually the colors are replaced by similar looking ones, which cannot be identified by our eyes.

Another way to compress is to reduce it's number of pixels(resolution).
If image is compressed by this way, every 2(or n)pixels will be combined to one depending on compression method. So, the final image will have 1M/2 = 500K pixels. Again, by this method also, the size of image is half.

In this, way the size of image differs.