First off, these things do not just happen, so what's changed? Find that out and it might answer your question.
There are three areas where filesizes can be limited, the Server, depending on the server, so that can be altered in the iis.ini file in windows Server, the httpd.config file in Apache to set the file sizes to a particular size. You will need to discuss these changes with your Systems Admin people to get these changed.
The second place to change is the php.ini file. the values of max_file_size, max_post_size and max_time_out need to be increased to suit your need. There should only be one useable PHP instance on your server so there it will be. But again, you may need to check with your Systems admins to get this checked. You can look at the PHP.ini file values by going to Site Admin > Server > PHP info, unless the System Admin have turned it off.
The last place is inside Moodle, there is a site wide setting in Site Admin > Courses > Course Default Settings and see what the Site Maximum upload size is. There is also a limit placed in all courses so in Course > Edit Settings > Files and Uploads.
That should cover it all.
AFAIK, Moodle has never had an issue with differentiating User uploads, so is there something the Users have done to limit their own uploads? Never heard of any such thing, but anything is possible, I suppose. Don't know about uploading from a phone though....mmmmmmmm