Site Upload Space Quota

Using WP MU (Multi-Site), I am trying to increase the available file space for a website.

I go to Site and then Edit, there is a field called, "Site Upload Space Quota". It is currently set to 1000MB. I can change it to 900MB and save it fine. However, if I change it to above 1000 and press save, it won't save the change and it will revert back to 1000 without any error message.

Is there another global option somewhere which sets the maximum size?