Why does he specifically say "non-trivial" defects? The probability of any defect surely increases with code size?

Perhaps to forestall the suggestion that the growth is in trivial bugs, such as a typo in the text printed in response to the -h option.

Maybe because these are the most relevant ones? Trivial defects are easily detected and fixed. Or else they're not trivial.

Of course. https://www.amazon.com/Economics-Software-Quality-Capers-Jon...