1) High islands sounds a bit strange XD lets call them island arcs,like the aleutian islands...

2) the closer they move to the subduction zone, the more activity produces new volcanic material... at least thats my guess...

once the arc reaches the trench, the islands will be sheared off like metal on. a truck that hits a narrow bridge. So no, i dont think it has something to do with buoyancy bit rather adding of volcanic material to the with and hight of the islands... often island arcs get sheared off and accreated (added, or pushed into) onto continental crust... in this process they get deformed heavily... the base of the island arcs, most of the time basalts and other volcanics, but possibly also igneous rocks) get subducted and either metamorphically altered or melted...

so its bot that they float somewhat, but rather get squished XD.