For bag work, you're probably looking at a 12oz glove at the smallest. You could go with the "Old School" slip-on type Mitt like this one here
, but I'd highly suggest a set of handwraps to go along with it, especially is you're doing a more intense session. Most bag work is done to increase hand speed in combinations and form. You're only going to work a large amount of power if you have someone holding the bag for you. If that's the case, you're going to need the extra padding and wraps to help protect your hand.
Most places will suggest 16oz as an "all purpose" size glove. A lot of places I know of won't even allow you to spar if you don't have at least 14oz gloves. As far as it being "too big" or "soft," I will tell you from first hand experience that you can still knock the crap out of your sparring partner with a 16oz glove. If you tag someone solid (especially if they're moving forward or coming back from a lean) the extra padding can make a difference between a black eye or bad cut.
Besides, when you're sparring you really shouldn't be trying to KO your training partner(s) anyways. Most professionals don't spar that hard on a normal basis to avoid injury before a fight. Sparring is there to sharpen your skills and technique against a live, moving target. In that case, the larger size helps protect from injury from any sort of incidental
contact. It doesn't gaurantee protection from intentional
The other reason why the large gloves are used for training, is to get you used to the heavy gloves and to build up your conditioning. As mentioned earlier, it makes it a lot easier to keep your hands up when you're using competition size gloves (10oz for Boxing/MT, or 4oz for MMA). Once the weight is shed, your hands will feel lightning fast. It's the same thing they did with training Kenjutsu techniques to Samurai. The training armor and bokken were much heavier than their actual combat armor and swords. As a result, handling the "live" equipment was less cumbersome in combat.