Final answer:
The characters blamed feeling sleepy after a big dinner on the heavy food they had consumed.
Step-by-step explanation:
Based on the provided excerpts, it can be inferred that the one thing they blamed for feeling sleepy after a big dinner was the heavy food they had consumed. In the first excerpt, the characters had a lavish breakfast after a hard night's work without sleep. In the second excerpt, the adults couldn't fall asleep for another hour due to a smell of cats in the night air. The third and fourth excerpts also mention difficulty falling asleep after a meal. These examples suggest that the heavy dinner contributed to their sleepiness.