I think the logo should be the same size, there is no reason to make it smaller as there is plenty of space, and while it looks okay in static screenshots, it looks ugly when it suddenly changes size.
I don’t know if it has been like for a while, I just noticed it now.
It has been like that since the beginning. As Keybreak already said, it starts with the main header image and when scrolling down, the forum uses the small logo image.