When you delete a backup you do not shrink the size the sparsebundle.. as I understand it.. more space is available but it is inside the sparsebundle and not released as free space on the TC. Or it simply takes some time for the space and sizes to be worked out.
You can do a disk image in which to store files..
http://pondini.org/TM/TCQ3.html
Leaving exactly the same size as the laptop hard disk for TM is not enough.. it does take a fair amount of space to work.. I would not allocate less than twice the actual size required .. which is less than the total hard disk space of course.
Please be aware.. TC die rapidly.. like one day it is off and will never turn on again. It is a sealed unit.. although you can get the hard disk out.. storing files on the TC is a receipe to loss your data. I have repaired 60 or more TC .. some I do just so people can get their files back. Remember you need another place to backup files.. and the TC cannot back itself up.. so you will need another tool.. it will be slow as it has to go through the computer, to wherever.. the worst being a USB on the TC itself. TM cannot backup network drives. So be careful.