I'm still on the 5 and was reluctant to upgrade mainly due to the size. Might consider this one when the 5 finally breaks down.
I expect a Plus has an even worse hold and I wouldn't be able to use it 1-handed at all.
This seems to make sense, intuitively, as well: I'm applying way more pressure to the bottom of the phone with a firm press of the home button than I do when I lightly tap it, even if I have to tap it twice.
Unfortunately, I found even the 4" too large for completely comfortable one-handed operation. Shame I'll never see another 3.5" iPhone option.