I've learned that "manually" saving by exiting to menu seems to work, however this is is a poor and temporary solution to such a major problem. My world size also fluctuates between ~30mb to upwards of 69mb.
By the way, I have 658gb free on my hard drive where Minecraft is installed.
Affects 1.9.0
Interestingly, this happened to me when my cousin had tried to join several times, and when she finally did, the game gave me this error.
Afterwards, 10mb was added onto the world file size, and any attempt the game made at saving had caused me to crash.
Turns out that this problem was caused by the beta resource pack for the new textures coming soon... can anyone verify this?
After deleting the resource pack the world was able to auto-save properly.