I'm not entirely sure if there is a pressure issue or not, but I have been down this road before with the fuel lines being different diameters in the past. What I did was put some boiling
water in a mug to warm it up and found a nail of appropriate diameter to push it on to in order to stretch out the hose. It took several tries of pushing it on and putting it back in the water to soften up, but eventually I got the end swaged out
enough. I also did the same thing on a car much more recently by heating up an old deep socket held in vice grips. If you use a nail be careful because they can be a bit jagged, so smoothing it out with a file, sandpaper, rock or whatever you have available will keep from cutting the hose open.