So-so I think.
The measle ball is not lighter. It weighs pretty damn close to the object balls. It may be heavier or lighter, based on the tolerances. And which set you use it with.
An oversized bar ball does not have a chunk of metal in the middle. It is intended to be used with a ball return system, and the way it is distinguished from a numbered ball by the table is its larger size. The magnetic balls you are thinking of tend to be the same size and weight (within the crappy tolerances of those ball sets).
I think what John was referring to though was differences between standard cue balls. For example, the red circle ball is a different composition than, say, the blue circle centennial ball. The red circle jumps like crazy.
I've never tried jumping with Snooker balls, but I bet it is very hard to hit the spot you are trying to hit with your jump cue tip.
John didn't mention, but the cloth makes a huge difference as well.
KMRUNOUT
You just lost every shred of credibility with the comment about being able to contact the necessary point on a snooker ball with a jump cue tip. JB makes more sense than you.
You need to look up the tip diameter posts regarding contact points. Tip diameter makes no difference as to striking the contact point necessary.