Large commits are when you have tons of changes that don't really all go in the same bucket. If I change the controller logic, then the database connection model, then some misc. scripts, I shouldn't be bundling it all under one commit.

Prevention is making commits according to what you're completing. In the above example I would commit after the controller logic, after the database work, and after the scripts. Don't put off committing simply because you know what changed. Other people will look back at your "Changed stuff" commit log message and wonder what you were smoking.

Initial imports are probably the biggest commits you should ever have. Setting up a system from scratch? Sure have a few big commits. After you've leveled it out though it's time to keep things organized.