But at the same time, you want to see all the content in the iframe. If you knew ahead of time exactly the layout of the text in the iframe you could do this, but it's harder when you have dynamically generated content inserted into the iframe, and now add to that wanting the page to be on different devices with different viewports, resolutions, users resizing the page, users increasing or decreasing text sizes for accessibility or changing default fonts.
And if you don't control the content, some of it may contain fixed size elements or absolute positioning inside the frame.
It's a really difficult problem that we were struggling with before ultimately giving up on trying to use iframes for this purpose. And when you make a mistake you either get ugly scrollbars in your iframe or part of your content is cut off when the user resizes the page.