Why my Reflection from scene capture cube is always 2~3 times larger than the real world

When I used scene capture cube to get the mirror effect to my StaticMesh in UE4, I found the scale of content in reflection (came frome a HDR picture which generate by scene capture cube and cube render target) ,for example,I put a box in front of the StaticMesh used cube render target map in its emessive,the reflection is 2~3 times larger than the real box. I wonder how can I control the size of the reflection? Thanks~

I also want to know!