AME is allocating huge amounts of space resulting in failed export

Hi all,

Hoping someone can help me with an explanation here. I'm trying to export a very large file (it's 9 hours long and the estimate is 42GB) to either of my hard drives which both have 100GB+ of free space, but once I get near the end of the encode my drives become full and the export fails.

Settings are pretty standard: h264, 10mbps CBR, aac 320 audio - screengrab attached. I know it's ahuge, unwieldy file but I need it as one file, not broken up into smaller chunks. I make these type of files semi-regularly and I've never had this problem before.

Can anyone explain why AME needs 3x or more space to complete this export? Is there a way around it?

Thanks