"Globe shrunk tight" could be talking about a microcosm which could mean the themes of the poem are reflective to the wider world.
"Round" could link with the title and theme cycles of death and rebirth because snowdrops are iconic of the spring time and death is very prominent within the poem. "Dulled wintering heart" is descriptive of the mouse and how it is dying and in the winter of its life. This also links with the seasons and death themes as it's the death of the year and the time of year when much wild life is either dead or dormant.
"Weasel and crow" are predators of the mouse and "moulded in brass" would make it seem as if the mouse cannot touch them or harm them leaving the mouse powerless. There are two comers on this line which slows it down and gives it a creeping feel as if the animals are hunting or being hunted.
"Outer darkness" the predators are forever in the darkness waiting to kill the mouse and other prey.
"Right minds" This could be saying that the mouse and other prey wouldn't go in to the dark in there sane minds. This is a short line and could be intended to be read fast this could represent the heart beating faster because the mouse is near death and struggling for breath or because a predator is near.
"Other deaths" could refer to the deaths of things in winter. "She...ends" could be talking about the mouse being ready to die or Mother Nature perusing the end of winter.
"Stars" this could be the stars of fortune and in winter the stars are brutal because a lot of things die and it's harder to survive.
"Pale" would suggest that either all the blood has gone or that the skin is malnourished because its winter. "heavy ... metal" this makes it seem as if its really hard to move maybe the animal hasn't eaten much because it winter.
Death, the cycle of life, nature, predator vs. prey, survival.
The done is very dark and cold words like wintering and brutal make the poem seem harsh and unforgiving much like nature.