The Revolutionary War angle is interesting, but I think it's hard to figure out the implications of that huge a counterfactual.

Would the Civil War have been prevented because slavery would have been peacefully abolished? Or would there have been a war against that abolition? Perhaps not extremely likely-- maybe the slaveholders would have been less willing to revolt against a bigger government, but it's hard to be sure. Could a larger block of slaveholders have done more to prevent British abolition?

I've heard that the British were more careful with their other colonies because they didn't want more of them breaking away.

WWI becomes really hard to predict. Would Germany have been more careful if Britain had been that much larger?

I admit I'm not sure if I'm arguing honestly on this. I may be assuming too much of a pull towards history as it happened.