I've been examining this before (I think I may have even posted it round these parts somewhere), but as I was just struggling with the Dunlop Jazz III JP vs. Carbon Jazz III, here's why.
The main reason why standard sized Jazz IIIs work so well for me, especially for funky rhythm stuff and other strummed things, would be that the pick doesn't stick out too much while the edge is still not touching the middle phalanx (is that the correct word? Seems so...) of my index finger.
Have a look, this is how much the Jazz III tip sticks out:
View attachment 51632
The pic doesn't exactly do it justice, but still. This is how I like it for very controlled strumming (on acoustics I allow it to stick out some more).
And here's how it's located between thumb and index:
View attachment 51633
As you can see, it's not touching the middle phalanx. Which is again exactly how I like it. As said, *the* very reason for me to use Jazz IIIs
Now, with the Jazz III JP, the pick's tip will either stick out some more (as much as the pick is larger) or touch the middle phalanx, in case I have it stick out less.
With single lines, it's actually absolutely fine for me when the pick sticks out a bit more, especially with the JP, as the pick glides through things incredibly smoothly. Allows me to pick pretty lightly, too - in fact, it lends to lighter picking automatically, which I like. But with most rhythm things I play, I can't play just *that* light (doesn't sound or feel right) - and that's the moment when the pick sticking out further really starts to annoy me. And it's getting no better when I pull it back a bit, because in that case the tip edge is "overlapping" into the middle phalanx of my index finger.
So, while the JP is fantastic for lines, not just because of the great shape of the tip but especially in combination with it sticking out a bit more, I'm almost sure I won't like it for rhythm guitars, even if I tried to get used to it. Btw., before I switched to Jazz IIIs, I used standard black Dunlop nylons (1mm) for years, so I do know I can get along with picks touching the middle phalanx, but for my personal feel, Jazz IIIs changed pretty much everything, so I won't go back there. And while I like me some effortless noodling, if I had to choose, I'd go for effortless rhythm playing any day (after all, it's what keeps getting me jobs, nobody likes noodlers - and I love it, too).
So, what to do? I will file one JP down to the size of a regular Jazz III and I will as well order some yellow Jazz III Ultex pack. Played them before and found them a little too slippy, but that was a long time ago. Their tip should come with somewhat less resistance than the Carbon things.
I may as well just try to polish the Jazz III a bit more, but I'm afraid that with the JP it's also about the bevel.
Never thought I would ever be geeking out on picks in my life again, but here we are...