I think you're going to have to give us more information than that. I might guess that your "including some dummy file operation" causes the Fortran run-time library to be linked in to your DLL, which would increase its size. Although, the default for building a DLL is to link to the DLL libraries which would not increase the size, but perhaps you changed that.
In most cases where adding something seemingly unrelated causes the results to change, the cause is a coding error in the application such as uninitialized variable, array reference out of bounds, etc. I suggest that you do some debugging of the problem to see where the "mismatched" results start showing up. This will lead you to the cause.
