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A Coffin — is a small Domain
Yet able to contain
A Citizen of Paradise
In it diminished Plane
A Grave — is a restricted Breadth
Yet ampler than the Sun
And all the Seas He populates
And Lands He looks upon
Breadth is defined as as a area of wide range or extant. A grave is visualized as a narrow 6x6 ditch. Emily Dickinson intends this line to mean the grave is physically minimal in space, but is spiratually and supernaturally limitless. 

A Coffin — is a small Domain