The SE is much easier to manipulate one-handed strictly because of the smaller form factor. Even with the 4.7 inch iPhone, I find myself using that more two-handed.
It should be obvious that a smaller screen is easier to use one handed. How much easier depends on the size of your hands; just print out an image of both phones and test for yourself.
I had iPhone 6, slimest device in iPhones lineup, could use it easily 1 hand. had iPhone 5, same thing. A bit harder with iPhone X, it's so thick, and slightly too heavy..
It’s definitely not too heavy. Good idea above to print out a to scale device or even better go to a store where they have them and try it out in your hand
I'm able to use my SE flawlessly with one hand. I'm was able to use my 7 and now X one handed too, but typing isn't as fluid as it is on my SE when I'm typing one handed.