Зернышко к зернышку - будет мешок

See Курочка по зернышку клюет, да сыта бывает (К)

Cf: A bushel of wheat is made up of single grains (Br.). Little and often makes a heap in time (Am.). Many a little (a pickle) makes a mickle (Br.). Many a mickle makes a muckle (Am.). One grain fills not the sack but it helps his fellow (Br.). Though one grain fills not the sack, it helps (Am.)