Deer will scrape away the snow to bed down. This may be what you are seeing. Check closely and sometimes you can see the imprints from where their legs are tucked up under their body. I saw many of these here in Iowa this past weekend. We had a snow storm dropping 14-16" of snow last week. We were pushing several woods during late muzzel-loader season.

I can't comment on the mattress sized one. That is pretty big.