Why do the size of the phase 1 notch vary among cardiac cells?
Between at least the endocardial and epicardial regions, I believe the transient outward current is facilitated much more in epicardial cells. This article demonstrates the differences in Ito between the two cell types in dogs.

I'm also recommending the following article about Ito phenotypes: "Transient outward potassium current, ‘Ito’, phenotypes
in the mammalian left ventricle: underlying molecular,
cellular and biophysical mechanisms"