Why are bars wider at the top than the bottom? Why is the wide part sometimes beveled on one side?
I assume you're talking about Hoffman style frames? If so they are narrower at the bottom so the bees don't glue all the frames together with propolis. None of mine are beveled so I can't speak to that.
The width at the top of the end bars provide the beespace between combs. they are narrower at the bottoms to provide space for the bees to travel thru from comb to comb
The bevel is suppose to help reduce and/or ease the removal of propolis. If you allow propolis to build up, it spreads the frames apart.
(http://www.bushkillfarms.com/photos/d/166-1/IMG_0760.JPG)