Unless they were far north (Upper Michigan, Northern Wisconsin, Northern Minnesota, Maine, canada, North Dakota) their body mass isn't the mst important thing for survival as food can usually be found. In the aformentioned states, they often yard up in thick pines/spruce/tamaracks for the winter to avoid the 2'+ of snow which saps their energy travelling in, and have really nothing nutricious to eat for at least 2 months.