Cassiopeia is a POISON mage
Can people learn the difference between poison and venom? It bugs me to no end when you complain about Cassiopeia being a poison mage that powers up.
If you bite it and you die, its poisonous. If it bites you and you die, its venomous.
So YES it makes sense if a poison mage powers up each time she uses it. The poison is becoming stronger WITHIN Cassiopeia. She is learning how to create better poisons within her body. If she was a VENOM mage then I would agree with you in how her abilities should debilitate your opponent. BUT SHE IS NOT.
EDIT: Wiki page on poison - http://en.wikipedia.org/wiki/Poison TEDTalk on Poison Vs Venom - http://ed.ted.com/lessons/venom-vs-poison-what-s-the-difference-rose-eveleth#review