Edit: Ah! I misread your post. Ok, so I think it's the way you copy the link?
Like, if you have the link to the series, it's going to go to the banner, like so:
But if you copy a certain episode, it gets kinda bigger?
And I think if you don't have a banner at all, it just defaults to the square thumbnail, like so:
There's also the fact that a lot of people do post their stuff on Webtoons as well, so their format is kinda different.
Is this what you were looking for? I guess if you want the thumbnail to be bigger for the overall series, you'd have to forgo the banner? But if you also publish your work on Webtoons, you get the default square thumbnail anyhow. That's how it looks like it's working.