We experimentally study the factors that influence the span in frequency combs derived from the crystalline whispering gallery mode resonators. We observe that cavity dispersion plays an important role in generation of combs by a cascaded four-wave mixing process. We observed combs from the resonators with anomalous dispersion and nearly zero dispersion at the pump wavelength. In addition, the comb generation efficiency is found to be affected by the crossing of modes of different families. The influence of Raman gain is discussed as well as the roles of the cavity diameter and pump power.