Try to keep your Tamagotchi at a consistent weight. When it is an adult, it's best at 30 or 35 pounds. If it's overweight, play games, and if it's underweight, feed it.
Basically try and keep your tamagotchi the lowest weight possible.
Since the weight won't go down any more that means it is the ideal weight. Just keep on playing games with it and continue keepng your tama healthy. Hope I helped a bit.