Large files upload

By default, the maximum size of a file that can be uploaded in Sitecore is 16 mb. In standard .net it is 4 mb. How did Sitecore achieve this difference?

Easy. The maximum size (or the maximum request length) is defined in the web.config. Look at the bottom of your web.config:

<system.web>
  <httpRuntime maxRequestLength="16384" executionTimeout="600" />
</system.web>

Altering the maxRequestLength will alter the upload size. The number defines how many kliobytes can be sent.