Diameter makes no difference. The centre of a small wheel turns the same speed as the centre of a big one. The only way you can change that is to turn it faster. It's like saying time changes when you look at a watch and look at the clock on the side of the town hall.
Leupolds take learning. Their characteristic is not to 'snap' in as much as other scopes which appear to snap into focus. This means they need care and consistency and time learning what their ability to resolve is. If you are focusing on something big or vague then that may mean it's visible and in focus for a wide range of distance. Focus on something smaller and it will be in focus for less. But because of the slow focus style of the scope you need to get there fast or else your eye's muscles will cheat and pull the image into focus before it actually is.
There are other scopes out there and not everyone gets on with them all to the same degree.
Objectively analyse your rangefinding capability with different techniques and find which one works for you. Prove it to yourself. It's the best way to learn, then you know what's best for you.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep