yacr85: again, you the have failed to address issue of the doctor himself performing the abortion. Possibly, Rav Leff’s intention may have been to follow Tosafos who holds that there is no heter of pikuach nefesh for a goy. So when faced with threat to the mother’s life, it would be muttar for a Jewish doctor and assur for a non-Jewish one. However, the “bigger” and “smaller” aveira is not relevant.
I’m eagerly awaiting your “sources” to permit abortions without threat to life. I have heard Rav Dovid Morgensten relate in HaRav Elyashiv’s name otherwise. He specifically permitted “reducing” a large number of fetuses only when the number would be potentially dangerous, and even then only in the first 40 days.