They aren't directly comparable. OS scheduling assumes 100% availability of the resources for its own purposes, with no external interruptions.
However, instead assume that the processor is only available for a couple 4-hour time slices a day. And, for reasons, starting a task that cannot be finished should generally be avoided. Now, if you need an estimated 4 hours to finish a task, that task must be scheduled at the beginning. If you take the alternative and schedule smaller tasks first, say they all take 2 hours. Well, you can't plug a 4-hour task into a 2-hour slot, so you just wasted 2 hours of time.
However, instead assume that the processor is only available for a couple 4-hour time slices a day. And, for reasons, starting a task that cannot be finished should generally be avoided. Now, if you need an estimated 4 hours to finish a task, that task must be scheduled at the beginning. If you take the alternative and schedule smaller tasks first, say they all take 2 hours. Well, you can't plug a 4-hour task into a 2-hour slot, so you just wasted 2 hours of time.