What’s your null hypothesis in this example? Statistically significant means the probability of getting a more extreme result when the null hypothesis is true is less than the chosen value of alpha (typical 0.05 is selected for alpha). In the case of this article, statistical significance means that if the two populations (treated and untreated) respond in the same way (I.e., no impact of the drugs) the probability of the observed differences in the sample (or more extreme - greater differential with few secondary cancers in the treatment group) is less than alpha. Sample size is also accounted for in the statistical test. All else being equal you need a larger difference between smaller sample size values to be able to conclude statistically significant difference.