According to this TibiaWiki page, the general formula for spell damage (in this case, healing) is
level/5 +magiclevel*x + y
in which x and y are specifics factors to each spell. For example, the minimum value you can get on a Mass Healing spell is x = 5.7 and y = 26 and the maximum is x = 10.43 and y = 62. You can do the math yourself or use the spell damage calculator from Tibia Stats.
Summing up the math part: you heal more (1 point each five levels) and your magic level affects directly the spell factor. For this spell, specifically, each magic level increases the Heal output on 8 points (on average)