Some people who rescue dogs are told the wrong information about their dog such as how big it is going to grow. Some people buy a dog knowing how big it will be and think it will fit in, other peoples circumstances such as living conditions change.
If a dog is not aggressive, then size should not be a major issue. Yes dogs can be excitable they can knock things over – but if they are walked every day and socialised, what is the main issue?
A small dog will usually suffer the same kind of mental issues that a large dog will if it is not socialised or exercised. Yes larger dogs may need to run more steps, but there are large dogs that inherently dont have a great deal of stamina too. They all need walks, but if a large dog is walked in the morning and has a yard or large apartment to walk in during thte day until owners return, then size shold not be a real issue.
Size becomes an issue mostly when a large dog is not exercised and it becomes destructive or barks a lot. Some large dog breeds are inherently docile, some our prolific hunters or fighters or guard dogs. You can only go against type so much. Man evolved these dogs and selectively bred dog breeds over and over to reinforce a given trait, try to make sure yours is one that can handle your lifestyle or the amount of time you can give to exercising it.
I meet a lot of people in the dog park that have large dogs. They love the size of their dog and dont have any more issues with it than people who have small dogs, because their dogs are social. Sure other park goers might get spooked to see their large dogs run towards them, if they dont know the dog – but if the dog is friendly there is no problem really. They know how much the dog needs the exercise and time each day with their buddies and they do it because they love their dog. Large or small, these dogs always are better to live with when you give a tiny amount of your time to them in what they really need.