The important bit is that the icon kind of defaults to rather large (512 px by 512 px). If I manually drill into the core-icon template and change the width:"100%", height:"100%" that's in applyIcon to width:'24px', height:'24px', everything is fine.
I could just make my own version here which sets the height and width to the added SVG item to iconSize instead of 100%, but I wanted to raise this as you work on 0.8 stuff.
Here's an icon I'm trying to import from an iconset.
The important bit is that the icon kind of defaults to rather large (512 px by 512 px). If I manually drill into the core-icon template and change the width:"100%", height:"100%" that's in applyIcon to width:'24px', height:'24px', everything is fine.