That is a very good question.

The answer can be simplified down to what is really two answers.

The first is: Overhead. When you download a file over the Internet, it’s broken down into many many little messages called packets, which you can think of like postal envelopes. Each envelope holds pieces of your file, but that’s not all they hold. They have to hold the senders address, the receivers address, your zip code, your town, your state, your country, etc. all the info it needs so the postal service (Networking equipment) knows how to deliver it. Let’s say the file you’re downloading is an image. The person sending it to you takes a piece of that image and puts it in an envelope and mails it. They intend to do the same with a million other pieces of the photo until you on the other end can take all the pieces together and look at the photo. This postal service of sorts will also put that envelope inside other bigger envelopes to help it move through the system, among other things. These encasing envelopes get put on and stripped off at various points during the journey, but each of those envelopes are tangible objects. Just because the envelopes themselves are not part of the image you’re sending/receiving, doesn’t mean they don’t take up space in the back of the mail truck. Also before they even started sending the pieces of the image, you and the person on the other end had to exchange a few letters agreeing to the image delivery. You also frequently send letters to each other to keep track of what was sent and what was received, etc to make sure everyone’s on the same page every step of the way, and not all of them contain pieces of the image.

To make things more complicated, “Downloading a file” can mean a great many things to a computer. In the case of the photo, what happens if some of the envelopes get lost? Luckily the postal service has a mechanism that helps the recipient notice that one of the pieces of the image you were expecting never arrived, so you can send a letter back informing the sender so that they can send that piece again. After all you don’t want holes in your picture. Let’s say that works — you now received that piece. Well that piece took up space in the postal truck the first time it was sent to you, and it took up space again the second time they sent it to you. There are other ways of downloading a file though where you and the sender agree not to resend any lost envelopes, you’ll just deal with the holes in the photo. This obviously takes up less space overall because there are no duplicate envelopes being sent, and this decision is made without consulting the postal service. It’s strictly a decision made between you and the sender.

The second answer, which we will keep the letter metaphor for, is that not every piece of mail you send out or receive relates to that image you’re sending. You get a bill from your doctor in the mail, Your birthday is tomorrow so you receive a letter from your grandma. All kinds of stuff. The postal service is delivering all of it to you and it all takes up space in the truck a long side the pieces of that image.

Now you paid for a finite amount of space in the mail truck, and during this time you’re filling that space you paid for down to every last inch. You filled it with the pieces of the image in the envelopes, the envelopes themselves, the envelopes that don’t have pieces of the image in them but are related to the image, letters and the envelopes that contain them that were mailed to you by your doctor and your grandma, etc.

You filled 100% of the space you paid for in the mail truck, but only 90% of that space is taken up by the pieces of the image. The other 10% is filled with duplicates to earlier lost pieces of mail, the envelopes themselves, and the unrelated letters you’re receiving from other people.

This is why the postal service advertises to you and bills you based on space in the truck, a known and quantifiable amount of space they can commit to you, and not by how fast that particular image or anything else will be delivered. The postal service doesn’t exist for you to send images, it exists to send ... stuff. All the stuff, it doesn’t care what. They give you the space, you decide what to fill it with. How much of that space is taken up by just your image pieces is dependent on the image your sending, the way that you and the sender agreed to send it, how full the image piece envelopes are with other sets of instructions meant for you and the sender that doesn’t involve the postal service at all that you stuff in next to the image piece, etc. That’s your prerogative. What the post office is concerned about, and should be concerned about, is that the space you paid for is available to you on the truck, and that the truck will successfully deliver your mail from your local post office to the next post office on the route.