It's a playful and informal way to say you feel physically strong, perhaps after a good workout or a healthy meal. It's important to note that "fuerte" in Spanish doesn't always mean "strong" in the physical sense. It can also mean "tough" or "intense" in other contexts.
So, depending on the situation and the speaker's tone, "Estoy fuertesito" might also mean "I'm feeling tough" or "I'm feeling energized."