The Earths largest phylum is Arthropoda including centipedes millipedes crustace

The Earths largest phylum is Arthropoda including centipedes millipedes crustaceans and insects. The insects have shown to be a particularly successful class within the phylum. What biological characteristics have contributed to the success of insects? In many science fiction scenarios post-apocalyptic Earth is mainly populated with giant insects. Why dont we see giant insects today?250-500 words