FFXI's Healing Magic Potency = +x% to healed HP.
FFXIV's Healing Magic Potency = +x Healing Magic Potency stat

FFXIV likely does not have a cap because the Healing Magic Potency stat is nowhere near as powerful as FFXI's Healing Magic Potency.

+74 Healing Magic Potency in FFXIV will not increase your heals by 74%.