If you lock the zippers using the large holes, the compartment can easily be opened enough to allow a hand to reach inside. Try it and you'll see what I mean. Few little zipper locks are very strong, but avoid the cheapest ones. They're too easy to force or to pick. Small combination locks are better since thieves are less familiar with them.
Some travelers use an electrical cable tie in place of a lock.
When looped through the eyes of a zipper, a thief must cut it to gain entry and you'll know he was there. Of course, you have to cut it yourself to get in. I prefer locks. Any store that sells sailboats can cut a length of rigging wire and weld a loop into each end to receive a lock. Even the best lock doesn't prevent someone from slashing your luggage, nor from stealing it, but it foils the opportunist, the sneak thief who has only a few seconds to get inside and doesn't want you to know he's been there.