What is the logic behind restricting the size of an element if there is enough empty place?

enter image description here

This is screen-shot of a Freshdesk's custom app. The app is located in an iframe. Though there is a lot of free space both horizontally and vertically, the iframe has a maximum height of 700px and that much width as in the screen-shot. You need to scroll should you have a large content.

The iframe and its parent are cross origin, and you have no control over the iframe's parent. This makes it very difficult to change the size of it if not impossible.

Now, I really want to know what is the logic behind this UI design.

Update

iframe has almost the same height as the from. And the rest is free space.

Src of img: https://developers.freshdesk.com/v2/docs/interface-api/

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