When closing a PIE viewport, we write the size back to the New Viewport Resolution setting in case the window was resized during play. However, we do not account for the window border, and write back the full window size instead of the viewport size:
The result is a PIE window that grows on each launch, even if the window is never resized during a PIE session. This appears to be caused by changes made in CL# 14058158.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-162908 in the post.