My Macbook Pro will be hotter if I am using the webcam. When I had Mountain Lion, the fan will speed up quite a bit when I Skype. Since I upgrade to Mavericks, the fan seems not to speed up as much as in Mountain Lion but the computer definitely heat up. Unless you are talking about a very high temperature, heating up when you use webcam seems to be normal.
Yes, because the video encoder needs to kick in to pack the moving pictures into form suitable for over-the-wire transmission and reduce required bandwidth.