Starship HLS can lift much more than 50kg, but since Congress/NASA requires Orion to be the return vehicle, the amount they can return is limited by that, which only has a 100kg payload return capacity (and presumably a chunk of that is going to be taken up by food, spacesuits etc).

Same with why each stay is going to have to be just ~1 week. Starship can obviously carry more than enough stuff for 2 people to live off for months. But Orion is only able to stay undocked for 21 days.