Many a little makes a mickle
Many small amounts accumulate to make a large amount.
A mickle, or as they prefer it in Scotland, a muckle, means 'great or large in size'. Apart from 'many a little (or pickle) makes a mickle' the words only now remain in use in UK place-names, like Muckle Flugga in Shetland (which amply lives up to its translated name of 'large, steep-sided island') and Mickleover in Derbyshire (listed in the Domesday Book as Magna Oufra - 'large village on the hill'). 'Over' and 'upper' are very common prefixes in English place-names, along with their opposites 'under', 'lower', 'nether' or 'little'. Examples of these are the Cotswold villages of Upper and Lower Slaughter, and the Hampshire villages of Over and Nether Wallop. The word 'much' derives from the Old English 'mickle' and has now almost entirely replaced it. 'Much' is also used in place-names like Much Wenlock, Shropshire (there's also a Little Wenlock, of course).
The proverbial phrase 'many a little makes a mickle' has now itself been largely superseded by the 18th century 'look after the pennies (originally, 'take care of the pence'), and the pounds will look after ('take care of') themselves'.
The first mention in print of what was undoubtedly an older proverb comes in a 1614 work by William Camden, with a rather desultory title - Remaines of a greater worke concerning Britaine, 1605:
"Many a little makes a micle."
In the next century it was taken across the Atlantic by George Washington, who included it in Writings, 1793:
"A Scotch [steady on George, I think they prefer to be called Scots] addage, than which nothing in nature is more true 'that many mickles make a muckle'."
The phrase's variant form 'many a mickle makes a muckle' is also sometimes heard. This 20th century version is actually nonsensical as it derives from the misapprehension that mickle and muckle, rather than meaning the same thing, mean 'small' and 'large' respectively.
See also: the List of Proverbs.