In her last movie for MGM, Judy Garland is defininately a bit on the chubby side. But in her production number, 'Get Happy', she suddenly slims down. Then she puts the pounds back on for the end of the movie. Apparently, 'Get Happy' was filmed much later than the rest of the movie.
The number for get happy was actually filmed for another Judy Garland movie "Easter Parade," but was cut from taht film and used in Summer Stock instead. This is the reason that she is slightly less chubby in this scene.
The "Get Happy" number wasn't even suppose to be in the movie. It was shot and added after Judy lost some weight. They wanted the end of the movie to be really good and it was actually her idea to add this song. You can find this exact information about "Summer Stock" and the "Get Happy" number in her biography titled, "Get Happy".
Get Happy was indeed filmed at a later date as Judy wanted to end her career at MGM on a high. She took time off filming to slim down for the number. It wasn't originally filmed for Easter Parade. A number call "Mr Monotony" was filmed for Easter Parade but cut and this is where Judy first wore the dinner acket outfit but obviuosly the audience didn't see it until Summer Stock