I think I had my translation going in the wrong direction. After a night’s sleep, I’m considering より as being “than” after all:
“Because the length of the beak and tail feathers are also included in the length, the length is much longer than when standing."
Thus:
「コテイペンギンの体長は１００〜１２０cmでも立ってる時は８０cmくらい」
“The Emperor Penguin’s body is 100~120cm, but when standing it is about 80cm.”
Perhaps?