Hi R58, welcome to the forums.
The 100% is in relation to whatever "container" it's inside of (most likely a div)
If you want it to get wider, you could try making the container also 100% - YMMV
If you want it to get more narrow, it might be the cell contents are "pushing". Or maybe the container has a fixed width?
Seeing a bit of the markup and CSS might help some.