I think shorten life is just a rumor that big mom have to scare people. Cause that is how big moms devil fruit works. If the opponent scared to big mom, big mom can easily take their souls. Thats why she created the roulete. She scare people by threaten them using the roulete. Thats why one of big moms son said 'no matter you spin the roulete, you are going to die anyway' (more or less like that).
My thought is big mom can get stronger by absorb all of them souls that had been taken. What i mean is like gecko moria absorb all of shadow from his thousan zombie and become giant monster (maybe)(just guess it)