The sample size was not huge but large differences between the groups in the test are statistically significant. If the two groups are similar enough, then the study results of the large differences are mathematically and scientifically meaningful ("significant"). 

The researchers said that there were differences of about 5% at most between the two groups. The outcomes which differentiated the two group like income, etc were much greater than 5% difference. These differences lead to the figure of about 7 dollars returned for every dollar invested in the preschool program. 

Group random  selection was criticised mainly because the control group had more kids with IQs less than 87. Also the claim that preschool teachers give more attention to the kids that they knew had low IQs. Researchers say that all kids were treated equally and the low IQS kids did not have special attention. It was a preschool and NOT a special ed program .