Unless a node is doing mining, why should it limit the block size? Block size is determined by what the majority of miners accept. Other nodes are just slaves of the miners.
It is called validation because if some peer you are connected to you is going to start throwing huge blocks at you then they are chewing up your bandwidth with rubbish (same as sending you huge txs with no fees, etc.).
Full nodes will necessarily perform such validation in order to reduce potential DoS attacks (am surprised that you hadn't already realised this).
For SPV usage there is much less validation going on (which is why SPV mining is such a bad idea).