Prior evidence concerning momentum in Australian equity returns has produced inconsistent results. This study examines the interaction between momentum and firm size. Specifically, we report that momentum returns are significant only for larger portfolios, and that this finding explains the inconsistent results of prior research. We demonstrate that momentum is present only in the top 500 stocks, and is most economically significant among the mid-cap stocks, which we call a relative size effect. However, the momentum returns are primarily generated from poor performance of the loser portfolio rather than any superior performance of the winner portfolio. In a more formal examination of the impact of size, we find significant exposure to a size factor among the combinations of size and performance portfolios. The strongest exposure to the size factor is found in small loser portfolios which also have the strongest exposure to market risk. In explaining the source of momentum returns, our findings cast doubt on the practical implementation of a trading strategy, and we suggest that successful momentum trading strategies are likely to realize ‘paper’ profits rather than generate real investment returns.