If Little Kitty, Big City generated the same gross revenue on the Epic Games Store, Double Dagger Studio would keep approximately $6,075,287 - that's $1,338,127 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $692,085. 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 $4,737,160.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.