You are using TC as a "live cd" there, a concept which does not exist on official Windows, so that does explain some of the confusion. The CorePlus iso is larger because it contains the installer, wireless drivers, and other things - they are not installed unless you check the box, so the resulting install will be as small as you want.
So in short: grab the CorePlus iso, start the installer from the bottom bar. Once there, please follow the install guide on our main home page if you have difficulties.
After installing, remove the CD from the VM.