The Highlands islands are controlled by their individual incidences in the CC configs. Although they can merge with each other and with "normal" landmasses, I always find the large majority are isolated in the deep ocean, as I intend them to be. Do you have a ForgeAmidst map and configs for large island blobs?
Edit: BoP will generate these monstrous agglomerations of Volcanic Islands, Mangrove Swamp, and Tropical Island (the "South Seas Complex". They have nothing to do with Highlands, and are caused by a long-standing bug in the BoP sub-biome generator. You have to turn off BoP sub-biomes to stop them.