Yes, attacks greater than 1/4 of your hp will knock you down. Some attacks will always knock down no matter how much damage they deal (epsilon rafoie, ep4 fireball, merissa stab?, etc.). Def and evp are kept low because A. they are way too weak to do anything anyway (need several hundred before it makes any significant change) and B. there isn't enough room in material limits and mag levels to get them as is. Generally you focus on the important stats first like Ata, Atp, and Lck, and then there is no room left for dfp/evp anyway.