I would say, if you can ride your trails in summer while sitting and spinning, then go 2t smaller for decent (groomed/packed) snow or 4t smaller for deep powder.
If you need to stand to make your climbs in summer, you want to drop at least 4t.
Also remember, if your summer bike has 29x2.2 or even smaller wheels, and your fatty has 26x4.8, the larger wheel is making every gear combo that much taller.
There are not many drawbacks to a smaller ring.
The lower top speed is not an issue in winter.
Smaller chainring allows for more clearance with frame and is less likely to catch on rocks and logs, or to catch your baggy winter pants or boots.
If you are racing and need to stay in the draft on fast descents, that would be one of the only times I can think of needing a tall gear.