I think there is just a lot of mass shifting in TF, I mean in AOE Optimus somehow fits within a banged-up cab forward truck, something they avoided in the first films because they made it clear that Prime could not fit within such a design, clearly by the fourth film Bay just doesn't care anymore. TBH it's absurd when you look at that truck to think there is enough mass to make Optimus, because in reality it would probably make a bumblebee sized bot, at least if you expect him to not be hollow inside.
So there's no point applying to much logic to it. I do agree with Jyn that BB is perhaps a touch on the big side, but it's not a deal breaker and no one outside of the us who read this thread is likely to ever notice, and again, who knows, maybe in some shots he was that size.
I still do wish he was dirtier though, he's just never that clean in the movies baring 1 or 2 shots, the uber bright clean yellow just gives it that toy look sometimes. it's the chest/face/wings that need work, the rest is fine. BB also has a lot of colour variation in his face, as per below pic.
At first I thought the yellow was not the right shade but I'm pretty sure Prime1 got it right since they would have access to the actual shaders used, I think the difference you see on screen is more due to Michael Bay 's Orange/Teal colour timing of the film, which gives BB's metal a teal tinge at times and makes the yellow more orangey.
So I like it, but think it could be improved, since we tend to expect perfection from Prime1.
