In combat, wizards have some significant advantages over martial characters (AOE burst damage, number of options), and some significant disadvantages (single target sustained damage, survivability). Focusing only on combat, though, misses a big part of the picture.
Out of combat, wizards are comparable to martial characters (except rogues) in terms of non-magical, skill-based capabilities, but they also have access to spells that can completely transform the state of an adventure (teleportation, scrying etc.). If I wanted to pull back some of wizards' power, my first step would be to reduce their number of skill proficiencies. That way, they'd be able to do powerful things out of combat in scenarios where their spells were applicable, but would be more reliant on skilled martial characters in other scenarios.