Volquez, who walked two and struck out four, finished this spring with a 3.95 ERA. He is notorious for having poor performances in Spring Training, having posted a 6.33 ERA last spring.
"Finally," he said. "I think I got better throughout Spring Training. I was better than last year. And the year before. And the year before. And the year before."
What's the difference this year?
"I have no idea," Volquez said, laughing. "Really, no idea. It doesn't matter what happens in Spring Training. I'm just trying to move forward to Opening Day. I think I'm ready."
Left-hander Drew Pomeranz, trying to work his way into the Padres rotation, struggled from the start. He gave up first-inning homers to Whit Merrifield and Cain, the latter a two-run shot.
Pomeranz worked three innings, giving up five hits and six runs. He walked two, threw a wild pitch and had three strikeouts. He threw 66 pitches, 40 for strikes.
"It was bad pitch selection early on from me," Pomeranz said. "After those first three guys, I kind of settled in and started making pitches. ... That's a trap you fall in during Spring Training -- trying to work on something. But you have to get guys out."
Pomeranz is one of three pitchers fighting for two spots at the back end of the Padres' rotation. It's unclear when the club will make that decision. If he isn't moved to the bullpen, Pomeranz is be lined up to pitch the Padres' final exhibition of the spring, Saturday against the White Sox.
"Obviously, I'd like to have another start, to not have that be my last one," Pomeranz said.
Merrifield, trying to make the team as the 25th man, also doubled in two runs in the second, singled in the seventh, stole a base and scored three runs.
Padres catcher Derek Norris homered off Royals closer Wade Davis in the sixth.
Royals up next: The Royals take on the Giants at Surprise Stadium on Tuesday at 8:05 p.m. CT in a matchup available live on MLB.TV. Right-hander Yordano Ventura will start for the Royals, followed by Danny Duffy and Chien-Ming Wang.
Padres up next: Colin Rea gets one final chance to make his rotation case Tuesday as the Padres host the Dodgers at 1:10 p.m. PT in a matchup available via an exclusive live audio webcast. Rea, the club's No. 6 prospect , is one of three starters still in the mix for the club's two available rotation spots. Rea looked sharp early in camp, but has struggled in his last two outings, allowing five earned runs in each.