Considered the \"assurance bucket\" analogy. Why are some of the buckets larger than others for particular assertions or accounts

Some of the buckets are larger than others because certain assertions will be more important or present bigger risks for some accounts than for others. For instance, existence (or validity) is typically more important for accounts receivable than it is for accounts payable. After the auditor has determined the risks associated with the assertions for an account balance, s/he can determine the size of the assurance buckets (i.e., how much assurance is needed) and then begin filling the buckets by applying the "audit testing hierarchy."