When active, the body creates heat which is maintained thanks to an outfit’s thermal insulation. If your clothing is unsuitable, the body will get colder and blood flow to the hands will drop in order to retain heat in the upper body. Gloves and clothing must be fit for purpose to keep your hands warm.
If you're unsure about size, it is always better to opt for a pair of hiking gloves that are slightly too big than too small, for greater warmth.