The problem is indeed all the wheelie bags. Which is why I don't understand why they chose to reduce the dimensions of the 'extra' bag. In my opinion, if you take a wheelie bag, your second bag should go under the seat in front. The bigger it is the less room you have, but that's the choice you make. What difference does it make to the space available if my second piece is bigger than the sizer but still goes beneath the seat? If anything they should be reducing the wheelie bag size.
They didn't *just* change the dimensions of the second bag - they did something altogether more sneaky - they swapped the bags around.
This is from a year ago:
And this is today:
Notice how the terminology has changed. The wheelie size has become the additional bag.
It's subtle but important and allows BA to insist that all wheelie size bags can get checked regardless of whether you have a personal bag or not.