Some social apps like Instagram can only show portrait videos. In case you stream landscape video (width > height), these apps will crop the video to make it fit the height of the screen, which results in very narrow frame. In order to optimize the video size, Loola crops landscape videos to square and pad it with blank spaces above and below. For YouTube, Twitter and Twitch, Loola doesn't crop landscape videos but rather send them as is.

In order to show full screen in Instagram, you need to set your video side to portrait 9:16.
In webcam mode, this is done simply by selecting a portrait resolution in Loola:

In RTMP mode, you will need to set the output size in your streaming software to portrait resolution. 

Here's how a landscape video will look in Loola and on Instagram (square crop):

Here's how a portrait video will look:

You could adjust the source size in your streaming software (OBS/Wirecast) such that a landscape video will fit the portrait output width:
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Alternatively, you could use your streaming software to rotate the video 90 degrees: