Comment # 25 on bug 973599 from Wolfgang Bauer

(In reply to Bj�rn Voigt from comment #22)
> # GTK_THEME=Breeze emacs
> menu looks compressed

Yes, GTK 3.20 broke compatibility with themes, and KDE's breeze theme for GTK
hasn't been updated yet.
See also Bug#975219

There's hope though:
https://phabricator.kde.org/D1664
;-)

For now, just don't use breeze, select a different theme.