Well there is a lip around the edge to keep the hose from comming oof under pressure. They are Metric sizes on the car but the silicon comes in standard inch sizes, so some of them are not quite perfect and have to be stretched over to fit. You cant use a size that is bigger and just clamp it down, it wont seal, its better to just lube it up and stretch it over.
STretching it over isn't that difficult, and it will be much better than trying to clamp down on an oversized hose. It may take a little bit of work and fenagling, but the standard sized Silicon couplers do fit. Just have a little bit of patience, it will pay off if you do.