FYI, only the top pulley is a jockey pulley. The proper name of the bottom pulley is a tensioner or toggle pulley. Increasing the diameter of the toggle pulley wraps wraps about 50 -66% more chain than increasing the length of the cage by the same amount. The actual percentage will vary depending on the cage geometry. Thus you can increase wrap without decreasing ground clearance as much (an issue with ATBs in off-road conditions). A dirty chain is also less likely to bind and skip on a bigger pulley. It also reduces the deflection of a longer cage. Finally, it looks different and this must not be underestimated.



Now this makes the most sense of all, and frankly since this is on a longtail cargo bike, anything that helps the chain behave is all good!