the biggest factor

She picked up the biggest one in the line up, tossed it about a few seconds, as if trying to identify the weight with her hand. After messing with it for a few seconds, she tossed it to the ground. Then she turned and walked to another group of rocks nearby. This time she picked the largest amongst the group. The size was thrice as big as a human head, with that size alone, the weight could already be determined just by looking. No doubt it weighed almost a hundred kilograms. Yet in her hands, it looked like it weighed nothing. She tossed it about for a few seconds. The feeling of the hard material on her fingers was just right, she wasn't an AI robot so she couldn't tell exactly how much the stone weighed but mentally she could guess it roughly. This was just about the weight of a full fleshed adult with fat and flesh filling every part of their body. She nodded and stood back up

It was perfect.