I'll preface this by saying that I'm using an older computer running WinXP, as I daily am required to use some historic software that does not work on newer windows platforms.

But on the list of airline forums, the British airways one comes up in a smaller font on this screen, as per the screencap below.

Pedant mode off

Are you subscribed to the BA forum?
For me subscribed forums have smaller text or maybe because read those forums often

I wasn't aware of the subscription thing at all. When I went to look at it, I have 300+ threads subscribed to, but unsubscrbing from all of them didn't change how the BA forum appears for me on the list.

I read Mwenenzi's question as asking about the BA forum itself, not threads (in or out of that forum). When I go to MyFlyerTalk, I see a list of the forums to which I have subscribed. (Since I'm subscribed to all of the forums that I see in MyFlyerTalk, they all have the same size font.) When I get a list of all forums in a catergory, such as Hotels, the forums to which I have subscribed are in a smaller font than the forums where I am not subscribed. Whether I am subscribed to threads in a forum does not affect the font sizing. In other words, subscribing to a thread in an unsubscribed forum does not cause that forum's font to be smaller. As you found, unsubscribing to threads in a forum does not cause the forum's font to be larger.

In short, it seems to be a feature, not a bug.