I'm not talking just about how many pixels. But reducing the number of pixels in a picture without changing it's physical size. You can have a picture that's 4X6 and 300 DPI which will have a large file size. Or you can have a 4X6 72 DPI picture that has a small file size. We'd prefer the 72 dpi pictures.
I'm sure you've noticed pictures that are physically small but have large file sizes on this forum.
I just tested a couple of pictures a 4x6 picture at 300 dpi (print quality) is 6 meg. The exact same picture 4x6 at 72 dpi (screen quality) is 365 kb.
Did you view the 4x6 picture at 300 dpi on a computer screen?
If it's viewed at full size it'll be bigger than the screen and you'll need to scroll to see all of it.
Do you have the ability to see the actual pixel size of the two versions? If so take a look.
EDITED TO ADD INFORMATION
DPI = Dots Per Inch such as a printer can print 1200 dpi or 300 dpi. Meaning the spacing between ink dots is decreased as the number gets bigger and dots get smaller. I therefore can print a picture of some size like 4x6 with different resolutions by telling the printer what to do.
Computer screen has a resolution of n pixels x n pixels. A digital photo is measured in the same way, by size (number of pixels high x number of pixels wide). Therefore for digital pictures the only thing that matters for size is pixels. The file size in a jpeg can further be controlled by the amount of compression.
I think a long time ago with dot matrix printers a lot of confusion was generated.