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).
Not a doctor (or even a biologist), but I wouldn't be surprised if it was a heavy lift to expect the immune system to break down a fruit-sized lump, or the lymphatic system to transport it away. So you may want to remove most of the mass physically, and just leave the immune system to mop up the remainder so it doesn't regrow.
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).