"Clearly I don't understand prestiging. Help me out please."

#PokemonGO: OK clearly, I don't understand the prestiging aspect of the game. I do know that generally, you want to fight the gym mon with something with a smaller CP and try and exploit a type advantage. I haven't quit figured it out and in some cases I fight the gym with something half the CP of the gym mon and only get +100 prestige. Any example yesterday there was a 628 Magby in the gym and I was trying to prestige with a 356 Horsea...100 prestiege...then I tried with a 413 squirtle 100...prestige. Then in some cases when I fight the gym with something larger I get more prestige....Or sometimes 100....It seems like a total crapshoot.What am I missing? via /r/TheSilphRoad http://ift.tt/2iXwyZo