Bigger audiences want you to be bigger. It’s almost as simple as it sounds; the bigger the audience, the more the following points are true.
Increase your energy
That means increasing your volume, but is more than volume. More energy changes everything including body language. Energy alone will take care of most of what follows.
Raise your average pitch
Raise your average pitch (higher voice) as you would if you were calling out to someone a bit further than the most distant person in your audience. Increase the whole pitch range of your voice (higher ups and lower downs)
Lengthen your pauses
Pauses are always important, but in a large audience they become vital. A large audience cannot easily hear wall-to-wall words and is quickly bored.
Exaggerate your gestures and expressions
Frown deeper, smile wider, raise eyebrows higher. For a very big audience, raising eyebrows may well mean spreading your arms.
But what if I have a microphone?
The surprise is that no matter how good your microphone and sound system, all of those points above are still true. A large audience has to see and hear you making an effort to reach them, not just relying on amplified volume.
Next time you see a good presenter with a large audience, watch how much they work the energy, pitch, pausing and body language. They speak almost as if the sound system was not working!