This size change understandably has people concerned; a common adage in a non-contact sport like baseball is that one can get "too bulky" to be an impact player. Here is where I must disagree, and I elaborate after the jump.
If it is functional muscle gained through strength weight training (as opposed to bodybuilding or another form of muscle-building exercise, such as military training or strongman training), the muscle that Trout has added to his frame could conceivably aid him in various departments. Obviously, with more strength and muscle mass comes the potential for an augmented home run total, but his ability to steal as well as his ability to make it through the grind of a full season could be enhanced with the increase in strength and muscle.
Most people have stated that it's likely Mike Trout does not replicate the production he showed last season. I don't know that it's a certainty by any means, but it is a tough scenario to replicate when you are the first rookie in MLB history to post a 30 home-run, 40 steal season (not to mention falling one steal short of becoming the third player in the history of the game to have a 30 HR/50 SB season, a feat only accomplished by Barry Bonds and Eric Davis) and the youngest player to ever have a 30/30 season. Were Trout not able repeat such a level of production in 2013, it would not classify his season as a "sophomore slump" as much as it would be a regression to the mean.
However, we really only have his minor-league numbers, his cup of coffee, and last season in the majors to base his numbers off of. Trout outperformed his minor-league numbers and his cup-o-coffee numbers last season, but the question is really whether that is the true expected rate of production or just an anomaly.
Mike Trout's numbers through the minor leagues: .342/.425/.516 with a .941 OPS, 108 steals, 23 HR, 134 RBI.
Mike Trout's numbers through one season in the major leagues: .326/.399/.564 with a .963 OPS, 49 steals, 30 HR, 83 RBI.
Mike Trout's numbers during his cup of coffee with the Angels: .220/.281/.390 with a .671 OPS, 4 steals, 5 HR, 16 RBI.
Considering his track record in the minor leagues prior to and after his brief 2011 stint in Los Angeles as well as performance at the MLB level thereafter, we can
One can see that Trout's average took a slight dip, along with his on-base percentage, when he made the jump to the majors from the minors. However, some of the doubles he had been hitting in the minors turned into home runs when he hit the major leagues, thus the nearly fifty-point increase on his slugging percentage and twenty-two point increase on his OPS (as well as a drastic increase in his home run totals at the major league level versus the minor-league level). This is where his increase in muscle mass and strength should be an aid; he should be able to muscle a few more pitches over the walls in major-league ballparks than he had been able to do previously and at earlier stops in the minor leagues. In particular, if his legs and core got significantly stronger, we could see his stolen-base totals stay consistent or perhaps even increase. His acumen on the base paths is already quite solid, so adding strength (and additional speed in the legs to go alongside that) could be beneficial.
We could see his production go down, sure; pitchers will now have a full season of at-bats to gauge him on, as well as plenty of video to see what adjustments he made over the course of last season in his batting style. More strategic employment of pitchers will likely be made by managers when facing Trout, as well.
At the same time, one has to take into consideration the fact that the Los Angeles Angels of Anaheim added more firepower to their lineup (potentially at the expense of their starting rotation). They lost Maicer Izturis to the Blue Jays, and replaced him by signing the premier hitter on the market, Josh Hamilton (late of the Texas Rangers), forming a lineup that features Trout, Albert Pujols, Hamilton, and Mark Trumbo, a very formidable group of power hitters. It will be harder to pitch around Trout (or for that matter, the top half of the Angels' lineup) than it could have been last season. Moreover, it is now primarily the Angels' offense that will be holding down the fort and be the key to success for the team, where it was supposed to be starting pitching last season. Obviously such plans could fall through or be changed at any given moment, but coming into the season that is the plan for Tony Reagins, Mike Scoscia and company.
For these reasons, I think we can expect a slight decrease in the slash line of Mike Trout, but not a significant one. His minor-league track record is close to what he has done in the major leagues, and with a full season (recall that he was not up with the Angels until May) of play, the overall volume of his counting stats should stay fairly similar to what they were, given that Trout is still not close to hitting the prime of his career.
Trout may very well suffer a horrendous season like he did briefly in late 2011; however, taking into consideration his track record, age, pedigree and surrounding lineup, I would not bet heavily on it.