InDesign CC 2019 (and other versions) has known bugs with its HTML preview renderer.
One issue is when an embed (e.g., YouTube) doesn't preview at the correct size. If you then export with in5, it will render at the correct size, but it will not match the InDesign layout.
Typically, InDesign shrinks the embed by about 25%.
To fix this issue:
- Start by selecting the object and choosing Object > Edit HTML.
- Then take the height and width and apply it as a style as well, e.g.,
<iframe width="XX" height="YY" style="width:XXpx; height:YYpx;" ...
Finally, click OK and that should cause InDesign to re-render the preview at the correct size.