Maximum JAR size is the largest size of JAR that can be installed to the device. So if your JAR is 400k, it cannot be installed to a phone with a maximum JAR size of 128k. The phone will reject the JAR, and will not even download it.
As an aside, 400k is HUGE... I know of no commercial Java game here in Europe of that size. Most GSM network operators here wouldn't even carry it, so far as I know.