ATEM Mini Pro ISO File Sizes

I have an ATEM Mini Pro ISO, and use it to record theatrical shows. Recently, I purchased a BMPCC 4K as a B Cam for my BMPCC 6K Pro. I recorded a show that was a little over an hour. That resulted in two MPEG files - one for each camera. What I can't understand is why the one for the 4K is 8GB, but the one from the 6K is 23.5GB.

I was under the impression that the ATEM would take any signal, and output a 1920x1080 MPEG-4 AAC H.264 file. So what account for this wild difference in file sizes?

Thanks!