Kacbu lists 1 Smartwatches that stands out for Thin & Light: OnePlus. This is not a ranking; the product is here because its editorial assessment found it strong for this use.
For portability what decides is weight, thickness and whether the battery lasts a day. A light body usually means a smaller battery and fewer ports; that is a trade, not a flaw. Look at the weight and dimensions on the product page, and note that the maker's figure for the lightest configuration may not be the one you would buy.
Each card carries what sets the product apart. Open the product page for its strengths, weaknesses and alternatives.
43.2 mm · up to 60 hours