To first order, you answered your question already when you suggested time being a factor. 'Hotspot' volcanoes like the Hawaiian Islands cool over time, and since rock becomes denser (and less buoyant) as it cools it subsides to maintain isostatic equilibrium with the mantle. Geophysicists generally use an exponential decay relationship to model this process over time - that is, the magnitude of subsidence rate is highest at the beginning and eventually approaches 0. Given this, you'd expect the size difference between the youngest and second youngest islands to be greater than the size difference the second youngest and third youngest, and so on (where size is the maximum elevation on a given island, or the volume if we treat it as a right circular cone.)
But there are also some poorly understood processes that could be influential - namely what it is that controls a new island being 'born'. Thinking about the process from a mantle-frame-of-reference, you can imagine the Pacific Ocean crust sliding over the mantle while a plume ejects magma hot enough to perforate the crust and gradually build up a volcanic island. Understanding how exactly the magma makes its way through the crust - which is the key to understanding how one volcano stops growing and a new one is born - has remained elusive. The small-scale heterogeneities in temperature, chemical composition, density, structure and probably many other properties render techniques like seismic tomography (currently the most effective tool for 'imaging' the crust) pretty much useless; the term 'mush' gets used often to describe the region adjacent to volcanic conduits.
You're right that time is a very important factor. But there are other poorly understood processes that are influential at least to second order.
