I suspect that the impaired splicing leads to a multitude of premature stop codons and stretches of RNA that don't lend themselves well to transcription and/or

I suspect that the impaired splicing leads to a multitude of premature stop codons and stretches of RNA that don't lend themselves well to transcription and/or translation. Longer proteins, have more introns and are therefore more likely to suffer from this issue, at least that is my guess.