Hi @Stefan_Stoyanov 
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I don’t have a clear answer but I know that since row details has been enhanced there have been some inconsistencies (leading to confusion) with the Font Size in detail views 
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Before, when creating a detail layout you could place anything anywhere in the layout and then customize font size, alignment etc…
Now, I think the section you choose in the layout (I.e.: Title, Subtitle, etc…) to display some content overrides the custom font size… At least, if you’re using a different “Quick Style” than the 1st one (from the left) in the list …
So, from your screen recording the problem might be that you have a bunch of content displayed as Title including the Hyperlink and Description …
Small add-on :