could be written on the back of a postage stamp en · X
Etymology
Due to the very small size of a postage stamp. First use appears c. 1905. See cite below.
Meanings
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(figuratively) Something that is very small or limited.
Something classical that Annie couldn't immediately place, which was hardly surprising given that what Annie knew about symphonies could be written on the back of a postage stamp.
2008, Sam Baker, Deadly Beautiful:"That's the answer," he announced gleefully. "What you don't know about chickens could be written on the back of a postage stamp.
1905, “The Bird And The Bride”, in Poultry, volume 2, page 32: