Audacity 3.1 introduced non-destructive editing .
I believe the way that this feature has been implemented means that if you split your initial file, you’ve doubled the size of your project in memory: each half contains a complete copy of the original (unsplit) file. 
This means project sizes can grow far more rapidly than with the previous (destructive) editing system.
If you start with a 2 hour file you could rapidly run out of memory … Cannot continue using Audacity in its current state - #14 by steve