I have a 3.5" DF up front and a 7" at the console. They both display the same image, but the detail is easier to make out. If I split the front screen between GPS and sonar it gets really small. ( I am getting a 5" Helix soon as BRM gets them in stock).
Also when you zoom in it is a lot easier to make out detail on the larger screen, but I think the real advantage is when you are viewing split screens with 2 or 3 images displayed at once.
I have my 7" on a Ram Mount and I turn it so I can see it up front and find myself turning around to look at it when I can't make out something on the 3.5 screen.
By the way, your avatar Scrat is one of my favorite characters. I went with my Grandkids and nephews to see the first Ice Age movie and I laughed more than any of the kids, mostly at that stupid squirrel and acorn.