>... that's it. No explanation [...]
The explanation is ~'it shouldn't overwhelm it'. Of course that's a value judgement. Like saying the front bumper (aka "fender") shouldn't be too big on a car; or a persons hat shouldn't be too big for their hat (tell that to JK, https://goo.gl/images/ztdusQ).
The implicit point is that the transom is ostentatiously large, enlarged beyond the reviewer's sense of propriety.
Surely there is nothing else. It's a judgement of style?
