The double-breasted jacket is an ideal option for almost anyone, but especially for plus-sized groomsmen. The extra row of buttons and forgiving style of jacket are flattering on larger body types, which your guys will appreciate when they're on display in front of a few hundred people.
Single-breasted jackets can be worn by practically anyone, but they look especially flattering on men who are tall and thin. Either style can be paired with a cummerbund or high-button vest, although some experts will tell you that neither is required. It's widely accepted that jackets shouldn't be left unbuttoned, so most accessories would be minimally visible, therefore rendering them, well, pointless.