I'm surprised more people haven't advocated for improving the opponents' defenses. Adding in better saves, spell resistance, spell reflection, and anti-wizard tactics will help make the other party members (appear) more useful. It's true they might encourage the wizard to adopt even BETTER tactics, like party buffing... but party buffing doesn't have the same "the wizard is doing everything" feel to it. When an unoptimized mundane suddenly becomes large and hasty, they're having too much fun smashing to realize how screwed they'd be without the buffs.

In a large group it's more important than ever for each party member to have their day in the sun. Keep track of who's doing the most and who's doing the least, and adjust your encounters to favor the ones getting the least action. Throw in additional non-combat encounters if need be. Maybe the rogue needs to disable some traps, or the monk needs to... meditate, or something. If the wizard jumps in to try to take over those things as well, then you know you have a serious problem.