Late to the party, but I ran into this problem with bringing screenshots into Word 2016.
The problem is that Word wants to bring your 96 dpi image into a 220 dpi document. (Your dpi values may vary.)
When, say, a screenshot is pasted into Word, it inflates the size by a factor of 220 dpi/96 dpi ≈ 2.29167. (I don't understand the point of this "feature", but this is what my Word install does by default.)
To counteract this, after having pasted the image in, right click on the image and click 'Size and Position...". In the Scale subpanel, set your height and width percentages to 96/220 ≈ 43.6436%. This will scale the image back down to its original dpi.
Bonus pro tip:
Wait until you've got all your (poorly sized) images pasted into your document; don't resize any of them until you're done writing the doc. When ready, resize the first image as described above and hit Okay. Then click the next image once and hit F4. F4 repeats the last process, so hitting F4 will now resize the next image to ~43.6%. Click the next image and repeat your way down the file.
PS: Incidentally, changes to File>Options>Advanced>Image Size and Quality>Set default target output to... do not affect how my install of Word resizes pasted-in images.