If Call of Duty: Modern Warfare 3 (2011) generated the same gross revenue on the Epic Games Store, Infinity Ward, Sledgehammer Games would keep approximately $31,840,358 - that's $5,305,962 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $4,205,503. Epic charges 0% on the first $1 million in annual per-app revenue and then 12% on the remaining amount. On GOG (30% cut), the developer would earn approximately $25,232,103.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.