What is it that distinguishes this unusual, Greek-originated word that means
a terse, pointed saying, embodying one important truth in a few words; a pithy or sententious maxim
from all other adages and aphorisms? Apart, that is, from that cunningly-placed first letter 'h', snuggled after that second 'p' in a manner that is the bane of many a spelling-bee victim? No - it is the use of the adjective 'sententious' in the OED definition. For an apophthegm is no common-or-garden maxim, like 'all's well that ends well'. Instead, to be a true apophthegm you have to add a bit of bite, a slight whiff of enigma. 'Every little helps' is one such, used annoyingly by a British supermarket chain, to signify - well, what, exactly?
That every little purchase helps line the pockets of the swells who own it? Every little apple helps keeps the doctor from the door? Every little visit helps make you happy? You puzzle over it - and suddenly it is an apophthegm. If you remember how to spell it.