Thank you for your post!
It does seem that the Divider works as part of the segments rather than separate of the segments, which would suggestively cause the segments to spread apart with larger dividers. I am able to reproduce the behavior reported above on FileMaker Pro 15.0.3 with MacOS and Windows. I will forward a report to Testing and Development for review and post back here when further information is available.
Testing is able to replicate the issue, and all information has been sent to Development for further review.