It's not just baseball. Everything has changed in the past 15 years or so. It's a trickle-down effect. Major-league teams are expecting bigger athletes, so the colleges are, too. High school kids are lifting year-round.
Nutrition is better.
High schools have trainers. There's so much more going on now. Baseball players shunned weights 30 years ago. Now, even high school players pump iron year-round in elaborate school weight rooms. But I think the most important thing is flexibility.