Heterogeneous wireless sensor network (HWSN) consists of sensor nodes with different ability, such as computing power and sensing range. Compared with homogeneous wireless sensor network, the design of protocols for such networks has to be more complex due to the energy constraint in order to prolong the lifetime of the network. As energy consumption mostly comes from the radio transmissions, bandwidth utilization affects the network lifetime greatly. In this paper, a novel lossless data aggregation scheme (Better-to-Fit) is presented for the clustering-based HWSN to improve the bandwidth utilization without introducing extra computation. The average performance is evaluated theoretically and validated by simulations. It has been found that the proposed aggregation technique can minimize the aggregation time under energy constraint and achieve promising efficiency in terms of bandwidth utilization.