Longer cages will increase capacity as Andrew says.
If I have or move to a long cage derailleur will one or a larger pulley increase capacity?
To be transparent - I have no experience with oversize pullies.
I'm offering a summary of: "Do Oversized Derailleur Pulleys Really Help?" parts 1 and 2. Both articles are interesting but part 2 addresses the derailleur capacity issue.
The 46 tooth large cog on the XX1 did not offer a large enough range for everyday riders, forcing them to choose between pushing their bike uphill or getting dropped on fast downhills. Problems were solved by introducing a massive 50T cog as a twelfth gear. This large cog requires a longer chain in order to make smooth shifts. Instead of increasing the length of the derailleur cage, Sram increased the lower jockey wheel size in order to prevent the cage to run close to the ground and be susceptible to rock strikes. My wild guess is that they left the top pulley untouched at 12 teeth to increase shifting accuracy.
So, SRAM is building derailleurs that have a larger lower pulley (14 teeth on the bottom, 12 teeth on top) to increase capacity. (Linked as an example, not an endorsement)
It was difficult to find information on people moving to a larger pulley to increase capacity. Most of the articles went on and on about energy savings.
As a very skeptical person I'd like more information before making a change on my bike.
One thing that makes me more open to the possibility of this working is my experience with the triple pulley Sun Tour derailleur in the distant past.
It's not exactly the same thing but the two bottom pulleys are a little like a single larger pulley. The triple pulley did increase capacity. It seems logical that if a derailleur can handle more chain it can handle more capacity.
So, given my skepticism I'd be willing to try a larger pulley solution if:
- My current derailleur was already a long cage.
- I could find a larger pulley or pulley/cage combo that was not too expensive
"Too expensive" would be a price that would make me feel bad about throwing the parts away if the experiment didn't work.
Like SRAM I'd only put a larger pulley on the bottom - not the top - for the reason Andrew gives in his answer:
It also seems reasonable to assume that increasing the size of the jockey wheel closes to the sprockets without changing its location would result in a smaller maximum usable sprocket size.
I'd say you'll get more capacity, but you may not be able to use a larger cassette to take advantage of that larger capacity.
Let us know what happens if you take the plunge.