Yes, frequency response at your eardrum is ultimately what determines soundstage, which is influenced by different measured frequency responses of headphones and indeed IEM's. So that would be the mechanism for some in ear headphones emphasising or deemphasising soundstage vs other IEM's. With over ear headphones is where I think the magic happens re soundstage differences between different models. In other words this relates to my observation that angled drivers or angled pads combined with no ear touching results in the best soundstage when all headphones have been normalised to the Harman Curve through EQ. In other words with over ear headphones I think there is a soundstage element that is present in the physical design of the headphone that isn't there in the same way with IEM's. But, at the same time, as I alluded to, frequency response at your eardrum is what matters, and different IEM's will do that too through whatever frequency response they have been designed with - measured frequency response on a dummy head definitely affects perceived soundstage - I've noticed that through EQ experiments with my own over ear headphones. But, still, I can't get all my over ear headphones to have the same good soundstage by using EQ - in fact only my K702 and HD560s stand out in this respect, and curiously they are the only two headphones of mine that have angled drivers or angled pads whilst having an earcup design that enables my ear to not touch the pad or driver at all - so it's like my ear is "floating in space" if you like. I will also say that my "ear floats in space" with my HD600, but the soundstage is super small on that one - and that one doesn't have angled pads or angled drivers...........so my experience tells me that for over ear headphones that angled drivers or angled pads combined with an ear cup design that makes sure your ear does not touch any part of the headphone is what maximises the inherent soundstage of a headphone. Sure, the measured frequency response through EQ changes (for example) will change your perception of the soundstage, but if you normalise each headphone to the same target curve, then my observation on improved soundstage through angled pads or angled drivers & earcup design so it doesn't touch your ears is what ultimately creates the maximum potential size of the soundstage. This is all very anecdotal, but indeed what I have learned directly from my own obsessive experience with EQ'ing & listening to my various headphones.
(The mechanism for this is I think two-fold, my own ideas. Firstly, I think the angled pads or angled drivers emphasise differences in your own anatomy or your outer ear, so imparting a certain individualised/personalised "EQ signature" to the incoming sound that is specific to your own anatomy that will ultimately shape the frequency response that arrives at your eardrum - thereby putting some of your overall HRTF into what you hear. Secondly, I think there is a psychological aspect of not feeling the headphone touching your ear which may somehow help in creating the picture that the sound is not coming from the stuff strapped to your head. That's my thinking on it re the mechanism for my observations.)