system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do pictures that have the same number of pixels have different file sizes?
gpt
It depends on the type of file.

But most file types that contain pictures are compressed. Some pictures can be compressed more than others - if there are large parts of the picture that are the same colour, for example, then it's not necessary to store the colour of each individual pixel, and space can be saved by storing only the changes from one pixel to the next.