Independent sample t-tests are used to measure the statistical significance of the relationship between variables when the study meets these criteria: "the independent variable has only two values, the design is between groups, and the dependent variable is normal/scale data." Sample size influences the t value used to measure statistical significance, so small samples require a larger t to be significant.
Citation: Morgan, G. A., Leech, N. L., Gloeckner, G. W., and Barrett, K. C. (2013). IBM SPSS for introductory statistics: Use and interpretation. New York, NY: Routledge.