Atomic-force microscopy measurements on KDP ${101}$ faces are presented which show that the terrace width $W$ on vicinal dislocation growth hillocks is nearly independent of supersaturation \ensuremath{\sigma} and Burgers vector $b$, in contradiction to simple Burton-Cabrera-Frank models. An analytical model taking into account the effect of dislocation cores on step rotation is presented which predicts a dependence of $W$ on \ensuremath{\sigma} and $b$, in good agreement with the measurements. Using these results, we rescale macroscopic growth rate data onto a single Arrhenius curve, which gives a value of 0.33 eV for the activation energy of step motion.