While you can reduce the tool bars in size they are often needed and re-sizeing them all the time is terrible for productivity.
UE4 http://puu.sh/ktIXt/6df41f8549.jpg
Hammer(old) http://puu.sh/ktIQK/1dcff2f208.png
Blender http://puu.sh/ktJdH/74aa097bfd.jpg
Photoshop http://puu.sh/ktINX/c2debaa468.jpg
So as you can see the working area in UE4 is very small compared to other programs
Hopefully that makes it clearer.
Edit: more words, also this no longer problem for me as I have a 39" 4k but for the majority of people this probably a problem and if epic wants to reach all the people it can it should look at addressing this
34% of steam users have a 1080p screen; http://store.steampowered.com/hwsurvey