Image file sizes surprisingly large

Hello,

In the images folder, the size of the following image files is surprising large:
1.) right-column-bg-40.png (40px x 1px) is 17,707 bytes.
2.) right-column-bg-350.png (350px x 1px) is 17,716 bytes.

Because of a color change that I made, it was necessary to create a new right-column-bg-350.png file. The new right-column-bg-350.png file I created was 134 bytes. Why is the size of some of your images so large? It is curious.

Regards,

Steve