this is normal. Most camera will do the same thing if you take video at high-res for 50 min. If you want it smaller, you need use lower resolution. Not sure if you can do that on iPad.
So if you're trying to broadcast a live event through an iPad or an iPhone with one of those free services that allow you to do this, then will it work with HD or do you have to lower the resolution. But I am pretty sure they do HD video broadcasting through such services as well. I presume the issue will be the WIFI connection will be the bottle neck I guess for HD video you will need wired broad band connection.