A Bug's Life

Just the same number of pixels isn't the only possible difference ... when I take pictures and I use different ISO settings, I get different image sizes, because (as L_A_S said) how much the image can be compressed for storage depends on how much detail there is, and higher ISO produces higher noise in the image and makes compression possibilities less.

OTOH, when you save a JPG you say what level of compression vs. image quality you want, and so you can save the same picture at 1.4Mb or at 400kb and the picture will look quite similar, but a lot of the fine detail has been thrown away at the lower resolution (e.g. if four adjacent dots of grass are four slightly different shades of green, then that could be compressed to one shade of green repeated four times, which takes less disk space to store.

Similarly if one is being stored at 256 colours or 65k colours rather than 16 million colours, then the file size will be smaller.

And if the image file stores thumbnails at different resolutions on one machine and not the other, then again the file size can be different.