Quoting tweeter:

Quote:
I'm confused about how dpi is relevant to compression and file size.

DPI obviously matters when scanning and if you want to print it it might matter also.

But once scanned the file i'm working with is just dots (pixels).  I resize a 1000x1400 image to 500x700 and it's 1/4 the original number of pixels, regardless of the DPI setting embedded in the file.

I just created two copies of an image and set one to 600 the other to 100 dpi and resaved them.  They were the same size.  I resized them to 500x700 and saved as JPGs and again they were the same size.

Am i missing something

I should have said ppi (pixels per inch) not dpi (dots per inch). Dpi refers to dots of ink per inch in printing whereas ppi refers to the number of pixels per inch in image size/resolution.