This is a casual look at the idea of headphone size versus the sound that this size portrays.  I am looking only at a small selection here, what I have available, but each are well respected products and each at a similar level of performance.  So, does size matter?
 
The contenders:

1. AKG K702 Fullsized
2. Yuin G1A Compact
3. Yuin OK1 Earbud

 

 
The first step is to bring the earphones onto equal ground.  All earphones were amped via a Practical Solutions XM5 amp with upgraded internals.  The sensitivity of these earphones are actually pretty similar, so the end result is comparable once good power is on tap.  As well, I applied EQing to each earphone.  The EQ is an electric EQ that's pretty much infinitely adjustable and allows a lot of flexibility when needed.  The goal of EQing is to get each earphone to an ear flat (my ears).  Below are the frequency responses I ended up with for each earphone.  These are relative to my ears, not absolute.  This is mainly useful for comparison purposes only as your ears won't hear the same as my ears.  My goal is to EQ each flat from 30Hz to 16000Hz as best as I can perceive.  Once I have an EQ for each, I can swap between the earphones and EQ settings and get the exact same response for each.  All differences will only be a matter of presentation, not response which points out a lot of the little differences between each.  Without doing so, the natural frequency response will be a dominant factor in what you hear and will hide the subtle variatiosn, so I avoid that when possible.
 

 
Brief description of each:
 
Ok1

1. Offers an aggressive, slightly edge presentation that is strong in energy
2. While energetic, lacks some weight in the notes and subsequent fullness/power behind the edge
3. Natural sense of distance and space, not forward, not distanced
4. Excellent sense of timber with the aggressive nature and air with healthy high frequency emphasis (even when EQed down)

 
G1A

1. Smooth presentation with thick but slightly veiled presence which unfortunately hides some of the fine details
2. Decently robust sound with a good bit of energy.
3. Slightly forward presentation but with a good sense of space of the recording area, a benefit seemingly common to thicker notes and good articulation
4. A soft, laid back treble response (mostly from the thicker presentation) and a well presented bass response from the thicker notes

 
K702

1. Mild edge and almost laid back presentation but with a broad volumetric range
2. Excellent breadth of note, natural, airy, and excellent sense of space yet very mild mannered/pleasant
3. Natural presentation with excellent sense of space and ambient sounds
4. Lots of micro detail and ambient sounds that show through while still being an almost at a laid back state in overall sound.

 
 
So what does size matter?  Well the above are more so individual traits, not so much a byproduct of size.  However, size does play its role. 

1. We do see that the smaller earphones do lack capability in overall output capability.  The K702 is superior in volumetric range where quiet is quiet and loud is actually loud in a strong way.
2. Big can sounds don't only come from big cans.  Even a micro earphone of the OK1 offers a lot of energy and aggressiveness and sounds big and powerful.  Stepping to the G1A and K702, there isn't an obvious difference in presentation relative to size limitations.  The OK1 does lack some body in the note that would otherwise make it come across powerful and full, but that is part of it's bright design.  It is also part placement in the ear as I could push the buds against my ears and gain that power, so it may in part be implementation.
3. Comfort is independent of the headphone size.  Proper design and support will play the bigger role in comfort.

 
 
 
Final thoughts:
 
Well each of these earphones are quite good in their own right.  Does size matter?  To me, I don't feel it matters all that much.  I do think size can be beneficial to ease of design and cover a lot of points with isolation, comfort, and be very good in driver design and component design without being constrained by sizing limitations.  To actually get raw power and output capability, we do need actual size and pure air movement capability, yet we also don't need big size to get big sound.  The volume capability is obvious with the K702 versus the other two options, however the OK1 is more aggressive and energetic than the others only lacking in thickness of note to actually create a big, robust presentation which is more a design choice than a size choice.  Often I have thought about stepping to full-sized headphones to perhaps gain aspects that we can't achieve with smaller buds or IEMs.  I often question the extent of gains, if any, stepping to a full headphone.  Now these are only a few options, good options but only a few.  I will agree there is definitely a scale of raw output.  This is obvious when we step to large floor standing speakers in the home or to large concert setups.  Size will always play a role.  From IEM to bud, to compact over the ear to full sized over the ear phones, the scale is there, but it may not be entirely necessary.  I do think that size allows engineering freedom.  There are certainly outstanding options in every size.  I tend to feel the choice in buying an IEM, bud, compact headphone, or full-sized headphone is more of a choice of wanting to buy a specific product.  If I want the HD800, I'm only going to get it as a full-sized headphone.  There are also a lot of really outstanding IEMs that offer aspects that can't be bought in a full-sized format.  In the end, you simply find the product that fits you well.  If that happens to be an IEM, it's an IEM.  If that happens to be a full-sized headphone, it's a full-sized headphone.

mvw2 you continue to post positive, informative, and well written posts and I thank you for that. Very good, quick read.