I just received a heated jacket liner from first gear and have a fit question. This is my first piece of heated gear and everything I read states it should fit "snug". My question is how snug? I can wear it with a t-shirt under it but it is tight across the shoulders and there is no way I could wear anything additional under it besides the t-shirt. I am thinking I need to go up one size but looking for advice from someone who has experience with heated gear.