For growing the qcow volume total space see https://github.com/docker/for-mac/issues/371#issuecomment-262826610 (the qcow location is now configurable in the Preferences of the app.
For shrinking the qcow file image size see https://github.com/docker/for-mac/issues/371#issuecomment-265525709 which is now in stable as of 1.13. Compaction is performed on shutdown and TRIM is done every 15m (unless you execute it manually, then it’s done immediately).