In my past experiences, jumping someone in their city makes you an enemy of the city. If you are there, then size simply matters not. You would be fair game for anyone around the city that feels the need to cast you out of the city. Attacking a citizen in a city and running back to home soil would also leave the same open for those of the city to follow you back and slay you.
It has been done to me, it has been done to others. Key point here is, if you go into an enemy city you can be killed no matter your size. Believe me, I get it from August and I'm not near his size but it is my choice to accept my fate when I enter SD.
Written by my hand on the 6th of Cloudburst, in the year 1183.