So for me everything works with the change I did for classes in class diagrams (to be reported everywhere), I explain :
The size in pixels of a diagrams depends on its format (default is "A4") :
iso A5 : 592 x 840
iso A4 : 840 x 1188
iso A3 : 1188 x 1680
iso A2 : 1680 x 2376
iso A1 : 2376 x 3364
iso A0 : 3364 x 4756
USA A : 864 x 1116
USA B : 1116 x 1728
USA C : 1728 x 2236
USA D : 2236 x 3456
USA E : 3456 x 4472
USA Letter : 864 x 1116
USA Legal : 864 x 1424
USA Tabloid : 1116 x 1728
And you have the landscape version for each. I fixed A0 size long time ago being the limit supported by graphic cards at this date, and I think it is enough large.
After that the size of an element like a class depends on what is written inside, and of course the size of a text depends on the used font and its size. The more you choose a large font the more for instance a class takes place in a diagram. This is enforced in your case by the fact the bold characters are incredibly large.
Under Bouml I use the default system font while it has codec, else I force the family "Helvetica".
I don't know what is the default font used by Qt on your system, I just know it has crazy bold character and it is very different than the font used under all other systems and releases of them I know, and worst you set a large font size with 11 pixels (whatever the real size of a pixel on your screen).
Except the change to bypass QFontMetrics
problem under Windows 8 I will make everywhere, the only thing I can do is to recommend you to use larger format than A0, and to use a smaller font size.
If you set the size 11 of the font to be able to read non bold characters and the problems comes because of bold characters I can modify Bouml to allow the user to choose the font family, do you think this is useful ?