By having both phones lit, that picture merely shows how much more screen S4 has compared to iPhone. Your eyes are drawn to the screens.
Yes the S4 is larger but the iPhone is not that much smaller with huge bezels and much smaller screen. To claim the S4 is gigantic is simply ridiculous, let along claiming iPhone the absolute opposite.
Yes and screen size makes a big difference because it shows the area you must interact with, and therefore stretch your fingers.
How are you saying the S4 is not gigantic compared to the iPhone? By a mere glance? If anything that's subjective. You may think that way, but some others don't feel the same way. So who's right? It's all subjective anyway.
So let's look at numbers then.
GS4: 136.6 x 69.8 x 7.9 mm
iPhone 5s: 123.8 x 58.6 x 7.6 mm
That's a whole 11mm wider, which means 1 cm more of stretching. Not to mention the smaller bezels of the GS4 means the thumb has to cover MORE area, especially if edge swipes mean your thumb hits the bezel and swipes from left to right.
The height is 13mm taller, and while you can argue the iPhone has a large bezel, let's look at screen size then:
iPhone 5 (x): 1.96 in
iPhone 5 (y): 3.48 in
GS4 (x): 2.45 in
GS4 (y): 4.36
Half inch difference in width and 0.88 inch difference in height.
Here's stats on male thumbs length
5th percentile: 44
50th percentile: 51
95th percentile: 58
just looking at those numbers tells you a loosely placed thumb doesn't cover the GS4. You have to stretch it to reach. Given the 50th and 95th percentile only has a 7mm difference, only a few mm makes the difference. Therefore, if someone finds the iPhone 5 to be the "perfect size" to grip, then likely their thumb needs to be 10mm longer to find the GS4 to be the "perfect size to grip" also. It's a few mm here and there if you have no perspective, but if you put it in perspective of thumb lengths and average stats, you will see there's some merit to the argument.
Let's not even forget that an increase in width means not only does your thumb need to stretch more to reach across the phone, but it affects your palm too because you're palming the phone. Your thumb can only stretch as far so that you're still holding the phone adequately. For example, if you're been swyping away near the bottom of the screen and now you need to initiate a pulldown from the top right of the screen? You likely have to shift the phone in your hand before you do that with a GS4-sized phone. If I hold my palm at the lower half of the phone and stretch my thumb to reach the top, then my palm has to give up grippability in order for my thumb to stretch out. That's why it ends up being safer for the phone and easier ergonomically to shift grip to an upper portion of the phone so I can reach the upper areas easier. I don't understand how people can just say its a few mm here or a few mm there and act like it's nothing. Just because those are small numbers in the grand scheme of things we normally deal with (miles and km in driving), doesn't mean its meaningless. Statistical significance be it 10 nm or 100 miles is still statistical significance.
I'm not trying to say the GS4 is too large for most people or too large for me. I can handle it, but just throw out opinionated statements that you find the iPhone 5 to be comparable to the GS4 and therefore all other complaints that the GS4 is significantly larger are invalid is ridiculous.