Most eviolite Pokemon are bulkier. There are only few that do not benefit from eviolite (I think Tyrouge is one of them) and are better off evolved for their defensive stats.
Max Defence: 196 (w/ evo: 294)
Max Sp. Def: 295 (w/ evo: 442.5)
Max Def: 251
Max Sp. Def: 350
HOWEVER. Snorlax has higher HP and better offensive presence. Munchlax is also useless if your opponent has Knock Off.
Essentially, there is little reason to use Munchlax over Snorlax unless your tier restricts you to (NU/ LC), or you just want something different.