This really depends on which elements of the triple constraints (time, cost, and quality) are valued most by the project sponsor. At the start of a project, its essential to determine the critical success factors --what does the project sponsor want to see in order to call the project a success. If its time related, the SPI is likely more critical. If its cost related, CPI would be more important. You are correct that for small projects, SPI may be more important, but it would be wise to validate that assumption with the project sponsor.