Measurements of soot concentrations in diffusion flames of ethylene and ethane are presented over a wider range of length and time scale than previous experiments. In the past, this type of data has been examined in terms of fuel to soot conversion fractions as functions of local flame variables such as equivalence ratio and temperature. Here, instead, we examine the total soot mass produced from flames of lengths 4-95cms and find that the growth region is described by a continuous function of time independent of flame size and local stoichiometry for the particular fuel. Soot mass flow varies as time to a power of 2-3 for the fuels tested. This result is considered in terms of gas-particle collision rates as the soot growth rate determining process.