I have recently had this problem with my ~6 lb. puppy. There were 4 geese in the backyard which adjoins to mine. My little guy was determined to get to those geese and squeezed completely under the fence and over into the neighbor's yard before I could run to pick him up. Thankfully, he scared the geese and they flew away. My neighbor had already lowered the fence gates to the ground, but there were a couple of sections of the fence that there was a small space between the ground and the fence (due to uneven ground). These were small spaces, but it doesn't take much for yorkies to squeeze themselves thru. We have now added chicken wire type fencing around the bottom perimeter of the entire fence. I have not had any further issues with my puppy getting out. He doesn't even try anymore (and the geese are still hanging out in the neighbor's yard every once in awhile.)
