The system:

Alienware Area-51 M5790 Laptop - Vista Ultimate
Intel Core Duo T7400 @ 2.16GHz
2046 MB RAM
Office 2007 Professional

The scenario:
On boot up the system is using about 45% of the RAM. Open Excel, open a spreadsheet (72 KB file size) that was created in Excel 2007 that consists of 3 sheets, (2 with very little content) and one with decent amount of content. The unusual part about this doc is that about 3 cells have in the range of 5000 words in them. The rest number from just a few to several hundred (total number of words in the file is around 22,000 words). After launching this and a few other windows including Word and IE it still is under 50%. But as soon as I start copying and pasting and entering content into it and working in the file, the used RAM goes up to 85% or more. And slowly over the next several minutes climbs to 95%+.

I thought this had something to do with the undo/redo but if I save and close the file but leave Excel open it is still over 90% RAM. Task manager shows Excel is using close to 1GB of RAM.

So my question is why? And is there anything I can do to not get my system resources all maxed out like that (eventually it does hit 100% and starts really going slow).