In studies of heterochrony, timing of development is generally measured relative to some intrinsic dimension, usually size, which serves as a proxy for time. A complementary approach is to measure timing relative to chronological time, an extrinsic dimension. Here, colony growth of a marine hydroid, Hydractinia echinata, is used to compare size- and age-based approaches to rate heterochronies. Colonies consist of feeding polyps; a basal, encrusting mat; and, in some cases, stolons which extend from the mat. Size is measured by the area covered by the mat and, if present, the stolons. Rate of polyp production was compared between colonies using clonal replicates grown in a common environment. Age-based comparisons employed polyp specific growth rates calculated over the same ontogenetic time interval for each replicate. Specific growth rates for colony area were also calculated; rates-ratios (polyp/area) provided size-based measures of polyp growth, analogous to allometric coefficients commonly used in studies of rate heterochronies. These comparisons show that size can be a misleading referent for between-colony comparisons because the rate of areal growth is not constant among colonies. Size-based measures of changes in developmental rates may provide insight into patterns, but not necessarily processes, of heterochrony. The implications of the pattern-oriented framework of size-based heterochrony should be considered in paleontological studies.