My ex was the same too. I'm more introverted and while small gatherings will always be my preference, big gatherings are fine as long as I know enough people. I met a handful of his friends in small gatherings and it was fine, but he would also keep inviting me to huge gatherings with people I didn't know and literally go and be a social butterfly all night. If I didn't literally follow him around all night, I was just left. Sometimes I was introduced if I was lucky, but even then no effort was made to include me in the conversation.
After many conversations about it, he had the gall to complain that I never went to his events with him. No, I'd rather spend my spare time how I want it, not being ignored in a room full of people who know each other well.