As Drake already mentionned, "milsim" is not about rules, size of field, restrictions, weapons, etc.
It is a mindset. To try, within the limits and restrictions of the "game", whatever they might be, to achieve your objective/mission/etc., not get caught and start again when the CO gives you another one.
I've had snipers under my command in a game where they stayed within a hundred feet of the ennemy command post for an entire day (took them 2-3 hours to sneak into position), feeding us intel. Numerous times they asked the permission to take down the opfor commander, and I refused each time. Did they complained ? No, they acknowledged the ulterior motives of the refusal and went about observing.
At the end of the day, they where beaming with satisfaction about a well accomplished task, without having fired one time. They knew they had been instrumental in our team's achievements.
That's milsim mentality for you. The rest is just props.