To say this is fortunate is an understatement.
My immediate family gets along great. I think mainly because my siblings and I got our really mean disagreements out of the way as teens.
Now everyone just snipes at each other. Sometimes in ways that seem mean spirited to an outsider (my wife still sometimes will gape at the way we snipe at each other - in her family this would lead to years worth of not speaking and grudges), but we all know the topics to stay away from. You know - the really hurtful stuff. The sniping targets the small funny/embarrassing stuff, a snarky act of love for each other.
> It's the closest family that actually is the problem.
In our case, it's the extended family. The increasingly racist and unhinged know-it-all uncle. The aunt who is both a pathological liar, attention seeker, and also furious at the world. The passive aggressive aunt who was married to a pastor, who can't stop telling people how they should be living their lives.
10 minutes with any of them, and I'm ready to go to the bar.
Yes, it's about family, and yes, I'd say you're very fortunate. I don't know how common "a family where everyone gets along" is, but in my personal experience, it's quite rare.
> Maybe you could downsize? Have Christmas with only the closest family.
It's the closest family that actually is the problem. My understanding of the phenomenon is that people organizing the event typically succumb to perceived (I'd say imagined) social pressure demanding it to be perfect, which immediately sparks conflicts - by the time of Christmas Eve dinner, everyone is wound up, if they haven't already snapped multiple times before. The dinner then obviously becomes a disappointment - instead of familial love, you get familial contempt. Attempts to counteract it become a case of "beatings will continue until morale improves".
And, of course, the more this happens, the more likely it'll happen again next year, as everyone expects to have a nervous and unpleasant day, creating a self-fulfilling prophecy.