File size changes during download

This isnt really a problem so to speak- I'm just a litte confused. When i download a file, the size of the file is proportional to the progresssion of the download until it reaches 10%. For instance, if the full file is 300mb, then when the file is 10% downloaded, it will be 30mb in size. Once I pass 10% download though, the file size is immediately 300mb. That said, the file isnt complete until the download is at 100%- its really only 30mb worth of the file. The issue i have with this is my internet is capped at 2gb. That means if i preview a file at 50% and dont want it, i've already consumed the full file size worth of my download quota. Is there a way to make the size of the file proportional to the amount downloaded the entire time, so that at 50% progression, 50% of the file is downloaded?
Sorry if this isnt in the right forum...

it's called allocation and has zero to do with your bandwidth usage. if you have downloaded 50% of a file but it displays the whole size on your computer it's just because the first and last piece of the file has been downloaded. One of those has the file info/header. Your bandwidth usage will still be 50% of the file.

Other download programs will allocate space on your drive right away.

thankyou for explaining that