Sorry, just thought this might clarify things (or not)
Here is the code for one of the labels:
<p style="text-align: center;"><iframe src="https://learning.wintec.ac.nz/mod/hvp/embed.php?id=1530601" width="800" height="625" frameborder="0" allowfullscreen="allowfullscreen"></iframe></p>
<script src="https://learning.wintec.ac.nz/mod/hvp/library/js/h5p-resizer.js" type="text/
javascript"></script>
<p><iframe src="https://learning.wintec.ac.nz/mod/hvp/embed.php?id=1530601" width="1119" height="865" frameborder="0" allowfullscreen="allowfullscreen"></iframe></p>
<script src="https://learning.wintec.ac.nz/mod/hvp/library/js/h5p-resizer.js" type="text/javascript"></script>
As you can see the first iframe is 800 wide and the second is 119 wide. However, after I save it and then check the embed code for each this is what I get:
These are, by the way, the same embedded item repeated and the original has the dimensions of the second iframe; the first was that size originally but changed when I saved the label. Why the second didn't is a bit of a mystery, frankly. I have tried using 100% for the width, taken out the resizer script as well embedding into the top of the section (as opposed to in a label) which resulted in a slightly larger display, but not significantly. Nothing has been successful so far and can't find reference to this behaviour anywhere else.
By the way, I have also changed theme to Boost but that had no impact.
Sorry for the essay!