Office 2013 occupying 4 Gb RAM.

I have an ASUS with 8 Gb RAM and with windows 8.1. I'm writing my thesis, and the word file now has 10 Mb, and as i'm scrolling down, the RAM starts increasing to 3, 4 even 5 Gb. I don't know what it is. I have already tried with office 10 and the same thing happens.
Any help with be much appreciated.
Sorry if i'm posting this in the wrong way.

José