If LEGO Harry Potter: Years 1-4 generated the same gross revenue on the Epic Games Store, TT Games would keep approximately $4,249,847 - that's $964,742 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $443,161. 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 $3,285,106.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.