My suspicion is that the size itself isn't the problem, but rather the time it takes to upload (which is a function of size). The connection times out and the page hangs.
The bug that I'm reporting is less about the error happening, and more about the lack of any error message being displayed. The JS on the client side is aware of the error, but does nothing with it.
I'm sure you're right. In the past Aldie has stated that he loaded massive files through the BGG interface when he was on-site with the servers.