Hi… This is a multiple of the number of CPUs:
Minimum memory is
256m * shared CPU sizeor2048m * performance CPU size.
Since cpus = 2 was specified in the [[vm]] stanza, that came out as 512MB.
(I would like more flexibility in this as well, actually. In particular, a performance-class Machine with only 256MB would be handy in the age of CPU throttling, for things that just need to wake up once per day, do a bunch of SHA256 hashing within a super-locked-down context, and then go back to sleep.)
Hope this helps!