Is a small text header over a larger text header appropriate as first content?

I have this page. This page is a combination between a "congratulations" and "here's something else to do" sort of page.

The very first piece of content (besides the header/nav) is a large block with a high contrast background that says looks like this:

Someone who has no business with the design, but has a loud voice is trying to suggest making the first line ("Your card is now active!") the size of the second line and the second line the size of the first.

I'd like to be able to prove to this person, without stepping over my boundaries that this is incorrect, but I can't find a source as to why.

Please remember, this is the very first thing a user will see. Besides the design not looking good and this certainly not working for mobile, is there any article anywhere that can back up my claims?