When an environment is built, some files are cached for future use. This can cause the build container to run out of space, which generates the "No space left on device" error.
The build container's free space is unrelated to the amount of space allocated to your app.
Try clearing your build cache with the CLI command: platform project:clear-build-cache and redeploying.
For more information on this feature, please see the following page in the Platform.sh documentation:
https://docs.platform.sh/create-apps/troubleshoot-disks.html#no-space-left-on-device