I think there are a number of things that can affect it. I forget the exact name of the property but there is an IIS metabase property I think that can limit file upload size or max request length.
There is also a setting in Web.config:
<httpRuntime maxRequestLength="1000000" executionTimeout="360" useFullyQualifiedRedirectUrl="true" />
You could try setting a higher number there, but it may also be enforced from machine.config
NeatUpload, which is what we use for uploading files in mojoPortal also has settings for this and you may want to review their documentation about Web.config settings.
Hope it helps,