The Per Era Modifier isn't that big. Even at deity level on the last age, it's only 24% more (1,30/1,05) war weariness in the last age than in the first age.
At (for instance) emperor level, the difference between losing a catapult (classical age) and losing a mechanized infantry (modern age) is only 11% more war weariness (1.18/1.06).
It does make a difference, but it's not huge. It's probably mostly the city sizes.
well, if 5 AIs are in the modern age, it's 5 times 11%,
55% is a lot, isn't it?