It can't be sheer power.  Look at what Hanna Asher is able to do.  She had even less training than Harry, yet her ability to use fire magic is far more focused and powerful in regards to effect, but not in the sheer amount of power Harry can throw.  Hanna only used fire magic, so her affinity with it must be extremely high.  She's sacrificed everything else in order to have that focus.  So Ivy might only have a power pool on the scale of the Senior Council, but in many ways her focus will be sharper than even they can do because she has literally thousands of years of knowledge of HOW to do it.