Searching ebay you often see 09 or 12 size selling at very low prices, but rarely 06 size...
Darren, Stew has hit it square on.
06 size is the standard that everyone has latched on to, mainly because it can be used in an entire range of holders , from say 8mm to 16mm shank size.
Going up a size requires larger tool holders. So the general manufacturers have stuck with the 06 and left the larger sizes to the big boys. So you just might find that the larger tips are cheaper because they are not as popular, but the tooling to hold them is a lot more expensive because they are only made by specialist suppliers.
I found that eventually the small chappies catch up and start to make the cheaper holders. A couple of years ago, the profiling tools all used a small tip, last year I found that larger tips and tooling are becoming available to the general market at much lower prices.
To get back to your original question.
The larger tooling tips will take much heavier cutter loadings. So that will allow production to be speeded up. But remember, your lathe tool holding will have to be beefed up to withstand all the extra cutting pressure.
I don't know if they would last any longer, because tip life is calculated by loading to the tip cutting certain materials versus expected minutes life at that loading.
Tip salesmen always quote faster cutting speeds (loading) and longer tip life compared to their competitors. So you might be seeing odd boxes of tips that have been superceded by a higher spec version.
Unless you are a real expert in carbide tips, you just won't know what you are buying. Some of the 06 tips I have bought over the years have varied greatly in their tool life. I put it down to that I am cutting the wrong material with the tip I have in the holder, but because there is no way of finding out what they should be used for, I just put it down the experience, and hope for the best next time. I find you can get 06 tips from as little as 80p each (bulk purchase of 100 tips) and as expensive as 6 squid each. You pays your money, you takes your chance.
John