Magic armor weight and effect on movement is determined as per the DMG 28 rule, not the PHB 36 rule. It reduces encumbrance by half (only when worn!1)), reduces bulkiness by one step, and reduces MV reduction by one step.2) Special, truly weightless armor could of course be created by the DM as a special magic item.
Even magic plate armor will magically fit any creature of the size class it was made for. See the armor size table.
Falls do not count as damage in the armor's “mode”, and so magic armor does not get a bonus against saving throws for falls outside of the standard bonuses for it being magic (+2) and its plusses.3)
Special UA armor like elfin chain and full plate is typically only available as magic items.Ex4)
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