I’ve done this tour a few times. They don’t weigh you, if you look like you might be over the limit they will ask how much you weigh. A pair of people who went on the same tour as us were definitely past the weight limit, they were asked their weight, clearly lied about it, and the tour operator warned them of the consequences if they are overweight what could happen. The couple shrugged and they went on the tour with us without incident.