Kruk managed to add size during the season, just not the kind you want.
In my experience, that is completely false. When I was in the Marines we would run 3-5 miles and do 20-30 minutes of callisthenics every morning, then we would go to the pool and swim 1500-2000 meters after lunch, then go to the gym after we got off for the day. I managed to gain significant size and weight during that time despite the calories burned (which I'm sure were substantially larger than playing a baseball game) during our daily routine.
It was mentioned on the ESPN broadcast of Friday night's game that ARod has taken Cano under his wing and they spend 1-2 hours in the gym following the game 5 nights a week. If you take that approach and workout hard (and eat enough to support it), then you will gain size and weight during the season.
P.S. to ARod2212: Generally, when a person loses a lot of weight, some of the weight gained by working out is consumed by the body. But, yes, any trained muscle retained will gain strength quicker.