Even if it were immoral to abort a fetus, the fact remains that living, thinking, feeling human beings were rounded up, tossed into camps or plantations, forced into labour, starved, tortured, and murdered. They were conscious and very aware of what was happening to them. They didn't just die -- they suffered.
It's not just the numbers that matter -- it's what happened to those people, what they experienced, what they thought, and the pain and fear the felt. To pretend that it's a matter of numbers and nothing more only serves to belittle the true horrors of slavery and the holocaust.