You are right about the arms part, everything was sliding! so I sent it back and I should be getting a sive 38 tommorow, my chest is a 32 and waist is 29 so I hope this one works!The back protector can for sure make up some space, depending how thick it is etc.
If the jacket is too big on your arms however, that can be a big problem. In the event of a crash, you may not get any protection from the pads as the pads can slide around, as it's loose. Your off a bit tight then too lose.
Oh, looks good!