As for constitution, the maximum health you can have in Minecraft is 10 hearts, without using a temporary +x max health potion. In Terraria, one heart is equal to 20 hit points, so Steve has a total of 200 hit points. The maximum health the player can have, without the temporary effects of the Lifeforce Potion, is 500. Even without the +100 boost that the Life Fruit gives, the Terrarian has twice as much health as Steve.
HOWEVER!
With the Sharpness V enchantment on a diamond sword, Steve does 14.25 hearts worth of damage. Converting that into Terraria means that Steve's sword attack does a whopping 285 damage without defense, and that's not even a critical hit! With a critical hit, Steve does 1.5 times the damage, leading to an ASTOUNDING 420 damage. (insert 420 joke here.) Adding in all the defense that you could possibly have, and Steve's normal attack would do 122 damage, and his critical hit would still do a brutal 257 damage.
BUT WAIT, THERE'S MORE!
The earlier damage calculations (420 damage with a critical hit) were made using Minecraft's critical hit, which does 1.5 times damage but does not ignore defense. If, again, we translated that factor into Terrarian terms, meaning that a critical hit does double damage, we would get an INCREDIBLE 570 damage, with no defensive items or armor. Since the damage is doubled after being reduced by defense, a critical hit from Steve's diamond sword would cause 244 damage to a player with all the armor boosts that they have, and even a player with the +200 health bonus provided by the Lifeforce potion AND the Life Fruit, would still go down in less than 3 strikes, assuming he got all critical hits, and less than 5 from normal strikes.
TL;DR:
Steve is no slouch when it comes to damage and defense. Even at cap health in Terraria, with maximum defense, you'd still go down fairly quickly. Also, with 96% damage reduction, your attacks would barely scratch him, even with weapons that do over 1000 damage with a critical hit.
Maybe it's because Minecraft uses percentages to reduce damage instead of points.