It is a tough work if you want to buy a pair of sunglasses those are totally fit you. Sometimes, you will find that the frame is tight for you and sometimes you find that the lenses are not suitable for you, especially the metal frame sunglasses. Now you find that the frame is tight, and it is totally common and also you can adjust it by yourself. First of all, if you are next to the shop where you bought the metal frame sunglasses, you can just take them to the shop and then ask the staff to adjust it. Then, if you think you can make it and adjust it, and then you can have a try, because of the characteristics of the metal frame, you can just bend the frame making it fit you. If you think you will break your loved sunglasses, you can just adjust the screw on the metal fame. Nevertheless, I do not advise you adjust you by this way unless you are professional.
Well, it seems that the sunglasses you have bought do not fit you very well. So, maybe it is a little bit tight in the first place. But maybe it takes a few days before you are fully used to wearing it and it would get a little bit loose so as to fit you. If this does not change, I recommend that you send them back to the store and ask for replacement. Anyway, we buy them for fun, not keep them if unhappy.
If it is too tight for you, you could use the screwdriver to make the screw of the frame side a little loose. You could do it easily. Then you could also adjust the position of the nose pad which will make you feel comfortable. You could also choose to let the client in the eyeglasses store to help you do this.