Shoutbox

I think it all comes down to resolution. A typical webcam has a significant lower resolution than a camera.

What Messenger probably does with sending video is taking the resolution it can support and cropping the rest. It does this inteligently and uses the middle of the image, opposed to topleft corner, because usually the subject you want to show is in the middle (thus nothing is zoomed in, the image simply has a lower resolution, but it is shown in the same space - it appears to be zoomed in - PS: this is also called a "digital zoom", but actually nothing is zoomed in; the individual pixels are just bigger).

Roughly the same goes for video call. Here it tries to push the entire image into the same space. Doing that it needs to scale down the image because the resolution is way too high, and thus the 'quality' drops.

Reason why Video Call is lower quality than Send Video is because Video Call is meant for viewing (video) and talking (voice) at the same time in a bidirectional way. The most important thing here is communicating (and thus voice). Video quality is therefor not as important compared to Send Video, where the video quality is more important (and voice is less), also Send Video is meant as a one-way communication. Remember that all the data needs to be pushed over the net (and MS' servers), the more data, the more traffic. To compensate and keep the amount of data flow roughly the same, some 'inteligent' decisions are made based upon what is the most important (and thus what needs the bigger slice of the data traffic flow).

Or something like that...

PS: Often people compare Messenger's video chat with stuff like Skype, and can't understand why MS can't do the same thing. But there are major differences in how those things work. The whole setup of Messenger uses servers for example, Skype not. Skype pushes everything it can thru your connection and thus it might slow down other (internet) stuff, Messenger tries to be more 'polite'. As with many things, different methods have both advantages and disadvantages. Nevertheless, MS does recognize that they need to step up a bit on video quality though (compared to competitors). But this is a slow process, as it also requires a lot of hardware change. In the next version of Messenger you'll already see some major improvements in this regards.