Hi GLL, Rback knows alot more about this than I do, but here's my comment:
The 60" bucket is not a LATER item, just a DIFFERENT item.
The 48" bucket on the Mahindra 2015 & ML 104 is designed as part of a total package, and for good reasons. 1)safety 2)overloading of components and 3)dimensions. Using a 60" bucket gives an unwary operator the potential to load much greater weights. And this may exceed the design limits of front-to-rear balance of the tractor. For example, loading the thing up with bark mulch would be no weight problem. (full bucket loads, by the way, can be either HEAPED or STRUCK; heaped being totally full to overflowing) However, with a full load of wet sand, it would be a huge amount of weight. The 48" bucket helps ensure the operator will not overload the tractor. All this stuff is designed by engineers to be a safe & workable system for the AVERAGE tractor driver.
Too heavy a bucket load may also over-stress hyd cylinders and loader arms.
Finally, the overall tire width of a 2015 is about 53 inches. The 48" bucket stays within that width. The wider bucket may not be as practical for manuevering, getting into tight spaces.
My 2415 with the ML 105 has a higher lift capactiy than a 2015, but still has the 48" bucket. I'm happy with it, even though there may be times when I desire more bucket capacity.........