did you look at the ratios? It is not "about right".
The iPhone se has a 60.8% screen-to-body ratio while the z5c has a 68.9% screen-to-body ratio. For reference the 6s has a 65.6% screen-to-body ratio and has a virtually identical screen with the z5c.
The 6s is 138.3 x 67.1 x 7.1 mm vs 127 x 65 x 8.9 mm for the z5c vs 123.8 x 58.6 x 7.6 for the SE while the screen sizes are 4.7" ,4.6" and 4" respectively (Note the height differences).
EDIT:
You can see the difference here:
http://www.phonearena.com/phones/si...Sony-Xperia-Z5-Compact/phones/10001,9611,9501
The z5c and the iphone 6s basically have the same screen, while the z5c is much closer in size to the iPhone SE.
Screen-to-body ratio has and always will be a poor way of determining whether or not a phone has an "optimal" size. The reason for this is that the height of the phone (which obviously affects the body size), is affected by things other than the screen size, such as the presence of hardware buttons or dual front facing speakers.
The proper measurement is the screen width to phone width ratio.
The 5se has a screen width of 49.8mm and thus with a phone width of 58.6mm the ratio is 0.85
The z3c has a screen width of 57.3mm and thus with a phone width of 64.9mm the ratio is 0.88
0.85 versus 0.88 is pretty damn close to "about right", however it is worth noting that the z3c is actually somewhat wide for it's screen size (it's the same as the 6s plus, which people often point to as an oversized phone) and there are similarly sized phones with slightly bigger screen (and thus better ratios), in particular the galaxy alpha and the moto x (2013). The moto x (2013) has a ratio of 0.9 for instance.
If the 5se had a ratio similar to the z3c it would be 56.6mm wide (2mm less, or 1mm on either side), if it was similar to the moto x (2013) it would be 55.3mm wide (3.3mm less).
btw, the s7 edge has a ratio of 0.94 if anyones curious (best I could find with a quick search).
Finally it is worth noting that smaller phones almost always have worse ratios, since there is a limit to how thin a bezel can be and thus the smaller the phone the bigger the bezels will be relatively speaking.