The blocks are already very well filled, even if they are not 4 MB. But this case only occurs when transaction data and signature data both happen to be maximally filled. And fortunately, that happens very rarely. Small blocks guarantee decentralization, so we should be happy with it the way it is. And yet we should also be satisfied that the network is being used. Any usecase is good and will sustain the network over time. The open market will take care of that. :)