Because letters are unlikely to weigh more than a couple of hundred grams - anything heavier would be classed as a parcel.
it is weighed in grams
A letter is typically weighed in ounces or grams, depending on the postal service. The weight of the letter helps determine the cost of postage needed to send it.
A pen cap is around 1 gram. 1000 mg can weigh 1 gram. 1 mL of water weighs 1 gram
Some are weighed by ounces but most are weighed by grams.
kilograms
7.5 grams
Paper is weighed in GSM (grams per square meter)
Individual grapes would be weighed in grams but bunches of grapes would be weighed in kilograms.
Will's fish weighed more. Jason had 2000 grams. Will had 2290 grams of fish.
Normally birds are weighed in grams
kilograms
Once the sheep has been sheared, the fleece is weighed on scales, probably in kilos and grams.