@Eric535 - Not sure exactly, maybe because sending to 120k is more intensive than sending to 60k ?
Do the email they receive differ ? I know larger emails will lead to higher cpu usage, so that might be one thing.
The emails are the same. Its just the list. I usually send out half of it on the first day which will run for about 9 hours. Then the next half for the next day which run for the same amount of time. Makes no sense.