If LEGO The Hobbit generated the same gross revenue on the Epic Games Store, Traveller's Tales, Feral Interactive (Mac) would keep approximately $4,598,429 - that's $1,036,042 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $490,695. 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 $3,562,387.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.