Garcia would run circles around a crude destroyer like Lejarraga (who, btw, has one of the best homefield advantages in all of boxing. his fights are nuts), but Spence isn't just power and a set of big nipples, that guy can really box. And he can cut off the ring on fighters and punish his opponents. He's also a superb finisher. Garcia's a good fighter off the back foot, he's extremely versatile and he does everything well, and he's quick, but the size is going to be an issue he just can't overcome.
I think Spence wins, but I also think Garcia's probably going to be the #3 guy in that division, even in defeat. That vaunted crop of PBC 147's is starting to show its age.