I don't think this as obvious as it first appears. Magic rarely benefits the peasants and they are way more likely to be eaten by a dragon or killed by marauding orcs.
Of course, it really depends on the fantasy world. For example, the lot of a peasant in Eberron is better than real life which is better than Dragon Age.

I don't know, I think the fantasy peasant usually lives a longer and healthier life than the medieval peasant. They generally don't get cholera and die at age 30 of explosive diarrhea regardless of how common magic is. It also depends on whether they're in a "Good Guy Kingdom" or a "Bad Guy Kingdom".

In a fantasy world, at least I could go on an epic adventure to save a princess and find treasures and stuff. In medieval world, man's gonna keep me down...

These kinda go hand in hand, don't they?

Is there really a difference?

Leprosy is treatable by modern medicine.