Get sub-clusters from hierarchical tree

Hello, I use function linkage to create a tree for clustering. To really make clusters, I use function cluster as follows:

NUM_CLUSTERS = 100;
cluster_indexes = cluster(tree,'maxclust',NUM_CLUSTERS);

This will give me 100 clusters. The second option is to use a cut-off threshold. In both cases, the function gives me the cluster indexes for data points. Now because some clusters have much larger size, I want to go down to the lower level and get sub-clusters with smaller sizes from a large cluster. How can I do that?