The immune system reacts to molecules on the surfaces of cells, not macro-level properties. And even if you can get the immune system to react to the tumor, it

The immune system reacts to molecules on the surfaces of cells, not macro-level properties. And even if you can get the immune system to react to the tumor, it's better if you've removed 99% of it, so that there's less opportunity for the tumor to evolve away from the immune reaction. (Or, in the likely case that you just slow down growth of the tumor a lot: better to start out at a 99% smaller size).