A working test today. A project running at around 420MB in 10.0.50.301, opened at 420MB and climbed as high as 11GB before needing to restart 10.5.10.120. At this moment, the same project is running at 4.5 GB. Earlier is was at 3.5GB. Seems to vary.
This is an known and already reported issue of Cubase 10.5.10 on Mac.