Currently Unity SDK has numerous issues. There is no saying when it will be resolved as I couldn’t get any information out of Google whether Cardboard project is a gimmick, or something they will actively pursue and evolve until they get Magic Leap up and running.
So UE4 has a good chance to get this chunk of “market” (I put it in quotation marks as I am not aware of monetization happening with Cardboard; although it’s a solid PR platform), if they get UE4 working with Cardboard with lens distortion compensation, chromatic aberration compensation (optional), AA (doesn’t work currently in Unity with distortion), and decent performance on Android/iOS (current top smartphones, like Galaxy S6/Edge and alike, with 4- to 8- cores). I don’t think deployment size matters as much, since there are almost 1Gb Cardboard “experiences” out there already.
However, looking at how many bugs devs have to deal with in 4.9 and the sheer volume of work ahead for core platforms, I understand why Google Cardboard hasn’t been pursued actively 