Why does my dog act different around each of us?

I doubt it, dog's don't perceive strength by size, they do it by smell and sound. A small dog will bark a lot (loudly) to give larger dogs the impression that it is more intimidating than it actually is. Dogs also don't have brains capable of many of the higher functions humans do, like having a concept of time or emotional imperative. Its more likely the dog has just learned based on conditioning from both of you, that it will be rewarded for behaving a particular way with each of you, and punished for acting another. What it judges to be punishment or reward may be the difference between why it will only behave one way around each person.