As distance west of kilauea increases the size of the islands generally appears to decrease. What is likely the reason for this trend?

The reason is two fold, the first is the active volcano of Kilauea implies there is a lot of heat under the big island of Hawaii, and that heat causes the island to be buoyed up. However, the other islands do not have active volcanos, are colder, and have subsided, the 2nd is, because the volcanoes on the older islands are dormant, those islands are and are no longer growing and are steadily eroding away.