File Size vs Resolution: They're Not the Same Thing

Resolution is the number of pixels in an image, written as width × height. File size is the number of bytes needed to store it. Compression sits between them, so a 4000×3000 photo can be 800KB and a 400×300 graphic can be 2MB. Changing one does not reliably change the other.

How compression breaks the link

An uncompressed 4000×3000 photo would be about 36MB — three bytes per pixel. It arrives as an 800KB JPG because the encoder found patterns it could describe cheaply: smooth sky, repeated texture, gradual gradients. An image full of noise or fine detail compresses badly and stays large at the same pixel count.

That is why the same camera produces a 1.2MB photo of a brick wall and a 300KB photo of an overcast sky, both at identical resolution.

Where DPI fits in

DPI (dots per inch) is a third number, and for screen use it does almost nothing. It is a tag stored in the file that tells a printer how large to render the image on paper. Changing a file’s DPI tag from 72 to 300 without resampling does not add a single pixel or a single byte of image data — it only changes the intended print dimensions.

So "increase the DPI of this file" usually means one of two things: raise the tag, which changes nothing about the data, or resample to more pixels, which does. When a print shop asks for 300 DPI, they are asking for enough pixels to cover the physical size at 300 per inch.

Which one is a form asking for?

Read the units. Anything in KB or MB is a file size rule and is satisfied by any file of the right byte count. Anything in pixels — "minimum 600×600" — is a resolution rule and needs real pixels. Anything in DPI is usually a print requirement and is best solved by rescanning at that setting.

A file size rule and a resolution rule are independent, and it is common to satisfy one while failing the other.