When you produce files for WinHelp use, Mif2Go replaces each of your bitmap graphics with a reference to a .wmf file: an external metafile named after the .rtf file. Mif2Go creates these metafiles, and embeds the bitmaps in them, so that the graphics are scaled correctly in the WinHelp file. If it did not, all the bitmaps would be rendered by WinHelp at screen resolution (typically 96 DPI) without regard for their original scale. It usually means the graphics appear at two to four times their original size; this is generally not what you want.
The same is true of .wmf files imported into your FrameMaker document. When you choose File > Save Using Mif2Go..., Mif2Go reads in any external .bmp and .wmf graphics, adds the FrameMaker elements such as callouts, and puts the results out in scaled .wmf files, which are referenced in the .rtf files produced.
Mif2Go's default is to wrap .wmf graphics in new .wmf files. If you do not want them handled this way, set [Graphics] EmbedWMFsInWMFs=No to use them at their native size. There is a risk when Mif2Go tries to embed .wmf graphics produced in other applications that the result will look very strange; in that case too, turn off the embedding.