I've never heard an audience member complain "The ideas in that speech were way too simple for me - I wish it had been more complicated". However, I've often heard the reverse. There are no prizes for getting long words into your speech. In fact, using strong, simple words is the best way to convey a message.
So you don't have to use long words when you speak. Most of the time, you can make your points well with short words. In fact, big words can get in the way of what you want to say. What is more, when you use short words in your speech, no one will have to look them up to find out what they mean. Short words make us feel good, too. A small word can be as sweet as a ripe pear, or as sharp as plum jam. Small words make us think. In fact, they are the heart and soul of clear thought.
Take a look back at that last paragraph. Did it make sense? Good. How many words had more than one syllable? None of them. See what I mean?