A lot of people keep their Tamagotchi's weight at the lowest possible, which for teens is 15 or 20 pounds. If you're worried about your Tamagotchi, just play games with it and it will weigh less!
It can weigh up to 99 pounds without any problems. Don't worry about it too much. I've gotten Makikos and Nonbiritchis even if they're insanely heavy even at the toddler/teen stages.