The industry standard term for paper weights is 'gsm' which means grams per square meter. This describes the weight of the paper, and not the bulk or thickness of the paper.
Glossy paper tends to be thinner, as the glossier the paper the more it’s polished, and ultimately the more compressed it becomes. This means that 170gsm gloss paper is thinner than 170gsm silk paper. Additionally, while paper weights will remain within permitted tolerances, the thickness or bulk of the paper can change significantly between manufacturers. This may give the impression that one stock is lighter than the other, when the weight is actually the same for both.