In "Stopping by Woods on a Snowy Evening," what are the snow flakes compared to in the poem?
The adjective "downy" does seem to suggest that the snowflakes are being compared to "down," which is defined as a noun meaning the soft, first plumage of many young birds or the soft under-plumage of birds. The word "downy" in this sense would mean "down-like." Goose down is commonly used for stuffing pillows and comforters. Eiderdown, the soft feathers from the breast of the female eider duck, is especially prized for such purposes. The ducks pull the feathers out of their breasts to make nests in breeding season, and humans will gather these nests to sell for very good prices.
The word "downy" also might have been intended to suggest the idea of "falling," or moving downwards, which can be felt throughout the poem, especially in its seemingly downward-trending rhyme scheme. The AABA rhyme scheme in the first stanza becomes BBCB in the next. The B in the first stanza is picked up and continues falling throughout the second stanza, and so on. But the main comparison seems to be between softly falling, extremely light snowflakes and the white down of birds.
If we wanted to stretch this comparison further, we might say that the word "downy" suggests that the speaker, sitting there in the cold, is thinking of his own warm bed with a down-filled comforter and a down-filled pillow.
You didn't specify which Robert Frost poem you were refering to in your question, so I have assumed you are talking about this famous Frost poem. Please respond if it is a different one, and in future, please make sure you give all necessary information in your question so we can help you better.
To be honest, the snow is not compared to anything in this poem. The references to the snow and how it covers the landscape are just description. We are told that the woods are "filling up with snow," and that the only sound the speaker can hear apart from the harness bells of the horse is the "sweep / Of easy wind and downy flake." Such an example of imagery conveys the sound of the snow falling and being blown around, but it does not compare the snow to anything. However, the snow is definitely part of the overall description of the woods and how beautiful they are, which makes the speaker want to stay in them in the final stanza:
The woods are lovely, dark, and deep...
The snow is part of the attraction of the woods, making the speaker think that there is nothing more he would like to do than to stay there.