We're seeing reports that the volume is lower than it should be when casting with Chromecast. The relevant team is aware of this and they are currently looking into it.
If you're experiencing this, we'd first recommend deleting and reinstalling the Spotify app and rebooting your casting device, router, and Chromecast. Make sure that all devices are up to speed with any software updates.
If the issue persists, let us know by adding your +VOTE and the following details in a comment below:
Your casting device and its OS version
The Spotify version on your casting device
The Chromecast firmware version
We'll be keeping an eye out for your replies, thanks!