Hi there,
i can’t see what the Theme would be doing to affect your video sizes.
Those are being displayed in an iframe which has a fixed width: 1250px and height: 703px.
The only thing the Theme adds that affects an iframe is this CSS:
embed, iframe, object {
max-width: 100%;
}
And that is there to stop an iframe from being larger then its container.
What makes you think its the theme – When the content has been added by the page builder and it would be its responsibility to make sure the iframe is responsive?