In a nutshell, "bitmap" means "consumes alot of memory". Of course, you
could Google it and get a more technical answer, but trust me on this one.
Since you have discovered a method you know will work in a crunch, do some
more experimenting. Try selecting the Excel cells you wish to convey to
Word, right click your mouse,>Format Cells>Border> and add some dark lines.
Then select the cells, right click and choose Copy. Then go to Word and
click Edit>Paste Special> and choose one of the "Picture" options. Your new
"dark" gridlines should appear and the associated graphic will consume much
less of your memory resources. Fool around with these options until you get
the proper mix-- it's different for every application and you just have to
keep at it until you get what is right for you! But work at it from both
ends- Excel and Word present options which can optimize the final appearance
in your document. I wish you the best of luck- let me know if I can help
further.