In my experience, any time you make a change to a bitmap image in Word, the stored file format changes to BMP which is a very inefficient way to store this information. Doing other image manipulations such as changing contrast can also trigger the format change.

If your two graphics were both BMP to start with I would expect little change in file size by grouping them. But if one (or both) was JPG/PNG/compressed TIFF then converting to BMP will require a lot more disk space.

WARNING: Maybe this isn't correct - see other posts below because actually testing reveals other things going on with grouped graphics.

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Andrew Lockton
Chrysalis Design, Melbourne Australia