So I have no idea what could explain the 1.6MB decrease in the exe
size.
Bisection via bootstraps lead to r108079 where the size decreases to 9,634 KB.
Backport fix for Bug#11989 from trunk.
* mouse.el (mouse-drag-track): Do not set the mark if the user
releases the mouse without selecting anything.
Bootstrapping r108078 yields emacs.exe with 11,062KB, bootstrapping r108079
yields 9,634KB. I did it multiple times to be sure.
Any ideas?