Right fielder Austin Slater, the Giants' No. 21 prospect, according to MLBPipeline.com, had a three-run homer. Designated hitter Ricky Oropesa had a solo shot and a bases-clearing double.
The Giants' No. 1 prospect, shortstop Christian Arroyo, slugged a two-run shot.
The Royals put up eight runs in the third inning to take a 9-8 lead, fueled by Moustakas' three-run homer. Moustakas also had a two-run double in the fifth.
In the fourth, Royals designated hitter Kendrys Morales hit a towering blast to right-center that cleared the stadium.