In this work, we consider the problem of partial cooperative transmission in a heterogeneous network with densely deployed base stations. The objective is to design downlink transmit strategies in a way that optimizes the system spectrum efficiency while using small size base station clusters for partial cooperative joint transmission. We propose a sparse optimization approach for this problem, by utilizing the fact that having small cluster sizes is equivalent to imposing a certain group sparsity structure on the precoders of the base stations. Specifically, our formulation is based on the maximization of a penalized system utility function, with the penalization term chosen to induce the desired sparsity pattern. We then propose two algorithms to solve this problem and demonstrate their effectiveness via numerical experiments.