Sure, most little dogs are savvy about keeping clear of tails that can do as much damage as a mace. Little dogs learn to avoid a flattened foot by scampering out of range when a lumbering pack member high steps in his direction. Most creatures intuitively look out for their own safety. We like to think they do. We hope they do. The thing is, sometimes we have to act to reduce the risks.
We've all heard horror stories. Small Dog Slammed in Sliding Door. Little Mutt Folded Up in Sofa Bed. The truth is, some small dogs don't seem to know they are small. They run headlong into the teeth of a big dog as they both go after the same crumb on the floor.
By the same token, there are big dogs who are not aware of the harm their bigness can pose.
In real life, no amount of instinct, easy going demeanor, or cleverness guarantees safety.
(First we'll state the obvious, don't slam the door without checking that the little dog is out of the way. Check under the covers for sleeping pets before you fold up the sofa bed. Don't allow your dogs to pick up stuff off the floor unless they clear it with you first). Now, let's talk about some ways you can mitigate the danger when big and small dogs live together.
First things first: training. If you have trained your dogs to obey simple commands like come and sit, you can teach them just about anything. For example, if your big dog uses body blocks (this is like a pick in basketball. The dog puts his body between you and another dog or person for gain. Gain, such as first to be petted, first out the door etc). You must let the big dog know he can't pull a pick on the small dog. Tell him no when he tries it. Reinforce the command by body blocking him. He'll quit using body blocks once he realizes they earn him nothing but censure.
Another good thing to teach your dogs is the "watch where you're going" command. When Mable the Mastiff was a puppy she often flung herself around heedless of other's feet. This command taught her to pause; to pay attention to what she was doing. How do you teach this, you ask? Say the command while interrupting the dog's action. Thus, when Mable was running willy nilly through the living room, she heard, "watch where you're going" and there I was blocking her path, forcing her to stop. This command also works when dogs are playing together and things get a little rough. If your dog knows how to pause he is less likely to accidentally hurt somebody. There is no reason small dogs can't be taught this command too.
The most helpful strategy for avoiding mishaps between big and small dogs is basic awareness on your part. If the 10 pound dog and the 100 pound dog both enjoy chasing tennis balls, play with them separately! If they both want the couch to themselves and they don't peacefully work it out by sharing the couch- don't let either one of them have the couch. An important caveat: if anybody snarls or snaps, consult an expert.
Usually big and small dogs can live together in harmony. Sometimes you just have to micro manage a little.