Create a small demo of UML in dia with a font attribute mix of italicized,
normal, and bold; sans and serif; and fixed and variable spacing.  (I can
provide an example, if contacted.)  From the command line, convert it to either
"eps" or "eps-pango".  The result shows some character strings in a very small
font, while others, even with the same attribute set, appear normal. 
("eps-builtin" ignores several of the attributes -- so it's not a workaround --
but at least is consistent.)

Saving the file interactively makes no difference.