Maybe this isn't a totally remarkable statement, but I think it's worth noting: all other factors aside, Starlin Castro could easily tally twice the number of hits that Carlos Pena does over the course of a season.
Starlin could double Pena's hits total.
Is that to say Castro is really awesome and Pena is really terrible? Maybe kinda sorta, but not completely. More importantly, they're different hitters stylistically: while Castro almost always ends his at-bat by putting the ball in play, Pena does so only half the time.
And when I say completely different hitters I mean it: among hitters with at least 20 plate appearances, Castro has the third-highest ball-in-play rate as of today, at 91.4%; he walks 3.7% of the time, and strikes out 4.9% of the time.
In contrast, Pena has the fourth-lowest ball-in-play rate among that group: he's walked in 16.4% of his plate appearances, and struck out in 37.3% of them, meaning just 46.3% of his PAs end with a ball being struck into the field of play.
From there, the math is straightforward: for every 100 PA, Castro will put roughly 90 balls into play. He looks capable of a .333 BABIP given his speed and ability, so he'll get 30 hits for every 100 PA.
For Pena, take his career BABIP of .280. If he puts 50 balls into play, 50 * .280 = 14.
Like I said, I realize this may not be a remarkable finding; we already know how these guys play. But one starter getting twice as many hits as another? I'm sure Ichiro and Jeter have out hit plenty of teammates, but on a per-PA basis, like this? I'm less confident about that.
Also, Castro = Ichiro or Jeter? Sweeeeeet.
I've gone ahead and done a little bit of research to try and see whether anyone has ever doubled up a fellow starter in terms of hit totals over the course of a season. It might not surprise you to know that it actually happens quite frequently, but it also looks like it has never happened on a per-PA basis.
From my casual clicking around, it looks like the lowest hits-per-plate-appearance rate in the expansion era belongs to Rob Deer. In his 1991 season, Deer managed just 80 hits in 539 plate appearances -- good for a 15 H/100 PA rate, similar to the pace Pena is on now. And indeed, one of his teammates doubled him up in terms of hits. Cecil Fielder had 163 that year -- but his H/100 PA rate was about 23, so not double on a rate basis.
Ichiro doubled up a number of his teammates in 2001 when he collected 242 hits in 738 PA, a rate of 33 hits per 100 PA. But even though Al Martin, Dan Wilson and Carlos Guillen failed to reach the 121 H mark, their H/100 PA rates were all above 16.
In short, at this rate, Castro could be the first major leaguer to ever collect twice as many hits as a teammate with the same number of plate appearances.