At first the build-ups in power would be incremental for the surviving versions of Yulaw because the power of the killed selves would get equally split among all the versions that remained. For instance, when the very first alter was killed, each of the remaining 124 selves became only .8% stronger than before.
The first noticeable hallmark can be drawn when 25 of his selves were killed — or only 100 remained. By this point each of the remaining selves have become ×1.25 times as powerful as they were when all 125 of the selves were still alive. Similarly, they’ve passed the ×2.5 mark compared to their original power when 75 of the selves got killed (50 vessels remaining), and so on — the less surviving alters remained the quicker power accumulation happened.
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So by the point Yulaw killed all other vessels except Law, they both were ×62.5 as powerful (had that much life force). And if Yulaw managed to kill Law as well that’d’ve doubled his power and made him ×125 as powerful as he originally was (when all his alter-selves were yet alive).