Thank you for reporting this issue. I've created an internal ticket and we'll investigate this further. Since we are using the planes we receive from ARKit, it might be that in some instances, the plane is not entirely on the ground. Because of this the augmentation will be closer to the camera, making it look bigger. We'll see if there is a better way to filter the planes.
Sometimes systems uses a different plane at some point after resetting, which is the plane on the real sofa. That is why sofas look huge sometimes. This only happens with IOS though. We using Intant tracking.
Unity 2019.1.12, wikitude plugin 8.9.0.
Here is a video - https://drive.google.com/file/d/1oysGCRt5vkKeL8KMTJOX_aR1yCUwU2iS/view