I have the same issue sometimes. In my case, I could replicate it 100% on one file but it was inconsistent on a virtually identical file. I also found the size error wasn't permanent -- I could save and reopen the file to restore the button's appearance. I could also create a new window and then discard the damaged window. I use a single monitor configuration.
For me, the button resized when I accessed the sheet's HPageBreaks collection. I was able to avoid the problem by temporarily changing the window view as follows:
ActiveWindow.View = xlPageBreakPreview
' do pagination stuff using HPageBreaks
ActiveWindow.View = xlNormalView