They are small because they are growing on bare surfaces, not even soil, so they're just growing on abiotic things like rocks, so the first plants are more like weeds, lichens, small mosses, etc... because these are the only types of plants suited to turn this habitat back into its original climax community through means like nitrogen fixation. Also, because there's probably no to little nutrients there which can cause nutrients to be a limiting factor to their growth and cause them to be stunted. Need me to elaborate more?
