Generally, the greater the risk of material misstatement, the greater the extent of substantive procedures.
For any one substantive procedure, the extent of testing usually relates to sample sizes (e.g. increasing the extent means increasing the sample size).
However, the extent of substantive procedures may also be considered in terms of: selecting large (e.g. material) or unusual items from a population; or stratifying the population into homogeneous subpopulations for sampling.
It is not unusual in many substantive testing approaches that all items greater than the materiality level (taking into account performance materiality) are selected for testing. If an error is found, that error is likely to be material to the financial statement assertions.