The ones I want to do on my iPhone.
This is usually because my iPhone dings more, and is connected to an always on network connection + phone network, so it's often the point of first-contact with any specific task/response/conversation/work unless I am sitting at my desk at home already.
And while I am someone who does take a laptop and an iPad with them in a backpack at lot of the time, that's not all the time. I'm not walking around the grocery store with a MacBook anything propped up on my cart. I'm not hanging from a strap on a bus or train with a laptop in my hands. And using a laptop on the couch usually makes my wrists hurt.
But what exactly productive tasks that you need the bigger screen? Because for any productive task I can think of that requires a bigger screen I would need a keyboard and a touchpad/mouse as well.
Here I am wondering why you are doing work while also grocery shopping.