Tap into 12AWG wire with 20AWG wire
In the amp-gauge table and many websites, the 12-gauge wire capable of delivering 20A. Check here
Usually, a Voltmeter is not drawing over 0.5A. and using 20ga wire is okay, tapping into 12ga wire is okay too.
But for safety reasons, you can fuse 0.5A or 1A fuse in between your 20ga wire to the voltmeter.

Image source: 12 Volt Planet - Fusing Guide
EDITED: adding picture of how to

0.5A. but I do prefer to tap directly to the battery pin because it is quicker for me. :)