Piano Forum

It would best to see a video on how you are actually doing it. However , off the top of my head I can make two suggestions at that part:

1) Since you are saying you are using the 4th on the Bb (which I would do as well) , and you saying that the difficulty in that the scalar run is un even is in the Bb to the A, not from D to Bb, it tells me that you are managing that fast scalar run at that part (as most usually stall at the thumb under when trying to get up to speed) ..... however maybe you are bringing the momentum of the speed of your hand going down from 2-1-4 - brings you to focus on not missing the 3 on the A ...and since the A is right next to the Bb, it is easy to miss since the hand is flying, so thus, you may be stalling there as you are "braking" your hand to  be able to depress the A.  Do you get the picture? Not sure I am explaining right.
You are throwing your hand so fast , that you want to   brake the hand to catch that A without just passing it by entirely. So knowing that, perhaps, make certain adjustments to your hand/scalar speed before you get to the Bb, to make it , let say, less un even.
So instead of : fast F fast D fast Bb abruptly stop for A...
you do : fast F less fast D less fast Bb pick up speed A...fast F... etc
so instead of a hiccup uneven, you have more of a ebb and flow , perhaps 'rubato' of that section of scalar run...and yes since that is incorrect, later, you can manage to tone it down and refine it so that the speed changes are worked in less noticeably and then more seamless eventually.
Not sure if that will work but it is an idea.

2) Scales are usually most done with the hands /fingers parallel to the keys by students but then we can start altering our approach to the scales by how the fingers are angled at the keyboard.
Let say in a chromatic run, there are many times when the fingers have to go 'perpendicular' to the keys a bit. ..kind of like a little two legs stepping on stairs kind of motion.
So, in the case of the Bb to the A, it is a short chromatic run, I would say try working the 4-3 fingers to do a perpendicular stair step down , that would perhaps accomodate your larger fingers if the standard way of scale play is making those two fingers flub there.