The problem, and the driving force of this argument is GWs lack of consistency in art. One image will them all manner of disproportionate then in others they seem half way plausible. Also people always make the mistake of their gauntlets and arm armor being incredibly thick while their big fuck off shoulder pads are thin.
Keep in mind their shoulder pads are like 6 fucking inches thick while most areas of their armor barely clock in at 1 or 2.
Kharn is infact the best example to show this with one arm in and one out of armor
Also there has been plenty of imagery showcasing the anatomy of an unarmored marine. Scaling also isn't hard. If 6 foot is the baseline for 40k humans. Space Marines clock in at over 7foot out of armor. Roughly 8 foot in.
Also here is some good material about the subject