There are a couple of reasons that a person might choose to use oversized outlet covers. They are used when the hole for the outlets was cut too large and for decorative purposes. There is no reason why you can't combine the fixing with using decorative switch plates and just make the mistake a design element.
The good thing about wall plate covers in general, and oversized switch plate covers in particular, is that they are a pretty cheap fix to a problem. Often when there is a defect in a house fixing it can cost a lot of money. So instead of stressing on the problem (if there is one) just be glad that the gap between your outlet cover and your wall is not going to break the bank to fix. Fixing that gap will also save on the electric bill as they help to insulate the house a bit.
There are a lot of oversized outlet plates to choose from when shopping. You can choose from oak switch plates, antique switch plates, or even ceramic switch plate covers. They can be plain and simple or as gaudy as you're willing to admit to being.
Oversized switch plates are easy to install or replace. Even the most inept DIY fixer upper can normally handle this task. It is as simple as removing a few screws. If you can turn a screwdriver you can replace an outlet cover. All you need to do is to remove any screws, remove the old plate, line the new plate up to the holes, and screw the screws back in. It's actually harder to find an outlet cover that you like than to install it once it is purchased.
When purchasing oversized outlet covers it is important (well a good idea) that you make sure that the plate matches all the other plates in the room. If you have antique brass switch plates in the room try to find matching brass outlet covers for any and all outlets in the room. The world isn't going to end if they aren't an exact match, but the room with look better if they are at least reasonably close.
Mixing different types of metal outlet covers in the same room can cause a serious clash of styles. Having a couple bronze wall plates and then a couple stainless steel outlet covers will actually draw the eye to your lack of attention to detail. I don't know about you but the last thing I need is someone noticing my decorating defects. So spend a couple extra bucks and get enough matching wall plates for the entire room.
Looks are important but if you have young children think about looking at some safety outlet covers to keep curious fingers from getting scorched. There is a safety reason that we use electrical outlet covers and young children is high on the list of reasons. The other reason that people should use wall plates is that having all the wires hanging out of the wall is a fire hazard. Keep them safely behind a plate to keep the home and family as safe as possible.
Investing in a few oversized outlet covers will improve the look of your home. They will cover construction defects and/or improve the look of a room. Large, dramatic, and attractive outlet covers can be a wonderful accent on a large boring wall.