Why a small port visit can carry the weight of a larger fleet

Does a small naval visit send a strong signal only if a large fleet has visited beforehand?

No. The other state needs to know that the larger fleet exists elsewhere.

A small visiting force can carry a credible diplomatic message when the recipient knows that a larger fleet exists elsewhere. The larger force does not have to have made an earlier peacetime visit. Its known strength provides the context in which the smaller deployment is judged.