Use of the subjunctive

[tacuba]

From a novel I'm reading (written in Spanish by a Spanish speaker).

"Porque él así jugaba, cuando el muerto era suyo o le parecía benéfica su desaparición, mandaba enormes coronas de flores, tan enormes que no cupieran por la puerta de la casa en que se velaba al difunto."

I'm wondering about the use of the subjunctive "cupieran". Is it obligatory here, or does it have something to do with the perception of the narrator. To me, it appears to be a statement of fact, that the wreaths wouldn't fit through the doorways, so I can't see why the indicative isn't used. Am I missing something obvious?

Thanks