Newark is completely surrounded by Fremont (California) and has over 40,000 residents and is over 13 square miles in area, so if it's not the largest, it's at least near the top.
If we discount natural borders like the Pacific Ocean, Santa Monica borders only Los Angeles and has twice as many residents.
Los Angeles has very strange borders. West Hollywood and Beverly Hills are independent enclaves, and it has an odd tendril stretching down to Long Beach. I remember being delighted when Google Maps started outlining city borders - I had always wondered exactly what LA's looked like.
Another fun one is San Diego, which does indeed touch Mexico, but to get from downtown to the border and stay within city limits you'll have to get on a boat and stay within a narrow band of water stretching down the bay to San Ysidro.
If we discount natural borders like the Pacific Ocean, Santa Monica borders only Los Angeles and has twice as many residents.