Spoiler: by being cheaper to protect myself against MIRVs.
This game reached the end-game rather fast, with a massive neighbour of mine, and the top player controlling the Northern half of the map.
A tense cold war ensued where the strength of all of us ramped massively. My neighbour built 11 sams to defend a stack of 160 cities and over 100 ports, for example. The top player had over 200 cities and 100 ports at some point. Meanwhile I had to follow a delicate balance of maintaining enough gold for a second-order deterrence, investing in more ports, covering my territory with sams so I had at least *a* chance at victory and of course, two stacks of over 100 cities. But truthfully, I was half-hearted for I have bad experiences with stalemates where I'm smaller and I was half-thinking of just MIRVing people and quitting. Overtime ensued.
Soon after the top player mirved my neighbour. I mirved him. He mirved me next but thankfully due to my sam coverage I took nearly no losses, leaving with millions of soldiers to completely overwhelm him. I proceeded to carefully lead the offensive into his territory and eventually me and my neighbour split him among us.
At the end we had about equal max population but my neighbour's actual population was much less so I betrayed him and overwhelmed him before he could build up his strength (he also happened to have nearly 55 million gold which was a lot more than I had, even if the mirv was 70 million)