This happened in my bathroom just now.

During construction, the builders told me, that the bathtub didn't fill out the length of the room. So they wanted to "pad" the adjacent wall a few inches so it didn't leave a gap. At the time it seemed sensible. In hindsight, it was a terrible idea. I'm sure there would have been better solutions for this little problem.

How can I best recover from this? And what did the builders do wrong?