My Jack Russell

Question
My Jack Russell, Rusty, is 16 inches tall and weighs 32 pounds.  He is not fat.  He can jump the flat face of a 6 foot fence. He is 7 years old. His mother was 13 lbs. and his father was 20.

My question is twofold. 1. How did he get so big?  2. Is there anyway to stop him from jumping out?  He may not do it for 6 months, but he may do it twice a week. He is an indoor dog with a doggy door and lives with a female German Shepherd and a Rotweiler.  They come and go as they please from inside to outside even when we are at work.  He gets plenty of exercise.  I don't want to put up an electric fence.  What are my alternatives other than leaving him inside during the day (which I've done before)?

Answer
Hi, Paul,

I can't answer the question as to why he is so big...possibly something further back in his pedigree may have influenced his size.

Quite frankly, I don't think there is anyway to keep him in the yard if there is nobody home to supervise him.  Behaviors are very hard to change if we are not present whey they are occurring, and every time he DOES jump out, his bad behavior is being reinforced.