I'll arbitrarily pick 100 audience members or more as a large crowd for the purposes of this article.
When you are speaking to a large number of people there are many different things you should know to have the most impact in the smoothest manner possible.
Speed of delivery
You will have to slow down your speed of delivery as crowd size gets larger. Even if you have a good sound system it takes longer for your voice and ideas to reach all members of a large audience. This is very pronounced as crowd size exceeds 1000 people. Also, audience members will be seated a great distance from you. Many depend on seeing and reading your lips to help make up for poor or declining hearing. If they can't see you as well, they won't get the idea as fast, so slow down.
Amount of material
Going along with your speed of delivery being slower you will need less material as audience size grows. This doesn't just mean you "need" less material. It means you "plan" less material so that you aren't rushed. You'll look really bad in front of lots of people if you don't cover what you promised them because you ran out of time.
A presentation to very large crowds in enormous rooms or venues such as stadiums means laughter comes in waves. The portion of the audience right in front of you will laugh first. Then most of the rest of the crowd will laugh. The third wave will come when those slower to get the joke finally do, and when those who laugh because everyone else is laughing kick in. You must allow time for this phenomenon to occur and be ready for it so it doesn't ruin your timing.
Also, in large crowds you must play to the back of the room. These people are hardest to reach.
In large crowds you can tell longer stories that you should never attempt in very small groups.
There are variables here. If you are not being I-Magged (projected on one or more big screens), your gestures and stage movement should be much more pronounced and broad. If you are being projected, then you don't want to be too broad because you will always be running out of the video frame and making it too difficult for the videographer to follow you.
Need for assistance
If you are selling products at the back of the room, it is imperative that you get some competent assistance. This really hit home recently for me when I sold about 60K at the back of the room. Even though I had offers for assistance from the sponsoring group, they would never have known the product and been able to concentrate on my stuff like my own assistant could. A good assistant will be an investment NOT an expense. I recommend Lorrie Morgan-Ferrero without hesitation if you need someone to help you handle a big event both in on site sales and in preparing promotional materials firstname.lastname@example.org
Super trick of the trade:
If you're really good, people will flock to talk to you. A good assistant will pull you away from the crowd to handle something, when really he/she is getting you away to give you a break.