Non-Euclidean Worlds Engine [video]
Prey (2006) modified a version of the Doom 3 engine to add portals as well as shifting gravity for similar effects.  I don't recall if it did size changes as well.<p>Demo/trailer
<a href="https://www.youtube.com/watch?v=zpQRePtEpRw" rel="nofollow">https://www.youtube.com/watch?v=zpQRePtEpRw</a><p><a href="https://en.wikipedia.org/wiki/Prey_(2006_video_game)" rel="nofollow">https://en.wikipedia.org/wiki/Prey_(2006_video_game)</a><p><a href="https://en.wikipedia.org/wiki/Id_Tech_4" rel="nofollow">https://en.wikipedia.org/wiki/Id_Tech_4</a><p>This tech predates Valve's Portal, although I doubt it is the first attempt at this.