Why do we say "Knee High To A Grasshopper"?

Meaning:

To be short, usually in the context of also being young.

Background:

The first known recording of this expression was in the USA in 1814 as knee-high to a toad. Other forms include frog, bumblebee, splinter, mosquito, jackrabbit and, of course, grasshopper which can be found in The Democratic Review in 1851:

"You pretend to be my daddies; some of you who are not knee-high to a grasshopper!

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