The RTF files Mif2Go produces can be quite large if the document you are converting contains bitmap graphics. You can minimize the size of RTF files by using 256-color bitmaps (instead of 24-bit true color), but even 256-color bitmaps do not compress well in RTF. However, once an RTF file is loaded in Word, Word can use internal compression methods to store the images more efficiently; then, when you save the file as .doc, the size will be smaller.
For files containing only text, the opposite holds: an RTF file is smaller than the .doc version of the same file.