copy + paste a picture in word document

I’m looking for and answer to why copying a picture from my file and pasting it into a word document creates such a huge file.

eg. i have a 3,800KB bitmap picture.
when i go and insert picture from file and select the picture the document is only 160KB.
but when i right click-copy it from file and paste it in word, the doc is 7.7MB.

can anyone enlighten me to why this is.