I think both of these players got feedback that they got pushed around a bit last season and wanted to make their bodies right (assuming they were going to stick with the same core).
In the case of Amar'e, he had no idea the Knicks would be trading for a top-5 center, so rightfully, he bulked up a bit to get his body ready for the physical toll of splitting minutes at the 4 and 5. As a result, he's probably carrying a bit too much extra weight - I'm a little surprised that his knee doctors didn't tell him that the bulk could impact his surgically repaired knees. Although alternatively, they could have told him to strengthen his legs so that the muscles would help take the brunt of wear and tear off the joint. (ie who knows)
Landry Fields came in knowing that he was going to spend the bulk of his minutes at the SG spot. Not sure why he got so big - but he's a young player, and he'll make adjustments physically. With the grueling schedule, I wouldn't be surprised that all of the players (especially Jorts!) look leaner as the second half of the season rolls around.