On Bob Villa show Sat. 3/16/03 concrete expert stated psi is determined by size of stone . I am under impression this is incorrect. That cement volume plus sand is strength determiner. help

The strength of concrete is the result of the varying amounts of cement, sand (fine aggregate), stone (coarse aggregate), and water used. Different mixtures will result in different ultimate strengths. No one single ingredient is the measure of the strength.

Not only the proportion of all the ingredients, but the temperature and what other additives were used are some of the variables that influence the final outcome.
The aggregate size is only one of the manyfactors but it is one of the three that are easy to control. The others are the amt of Portland used, and the water in the mix.