Why?

I don't see what the correlation is between a user's file size and how long the user will need it to remain.

Generally people use these services to host a file for around the time it takes someone to check their e-mail/ticket/etc. to download the item -- hopefully within a day, usually a couple days, worst-case scenario a week.

The time-cutoff is based on human behavior.

Plus, it's too much mental overhead for a user to have to worry that each file uploaded lasts for a different amount of time.