Because I've tried both. I tried on my buddies 37x12.5x17 before getting my 37x12.5x15 purely to check for clearance in my fenders. His setup was a perfect match for my 5.29s. My 15s have been dogs with a 37" tire. However I get a lot more sidewall wrapping around rocks so it is a give and take.
Are referring to the MTRs on 17 inch wheels that you borrowed for the clearance testing? I'm fairly certain that you're talking about your Pit Bull Rockers on your 15 inch wheels, right? If so, here are some relative weights:
Goodyear MT/R
35x12.50x15, C, 34.8", 67 lbs (+3 to 17)
35x12.50x17, C, 34.8", 64 lbs
37x12.50x17, D, 36.8", 73 lbs
BFG KM2
35x12.50x15, C, 34.8", 62 lbs
35x12.50x17, D, 34.8", 69 lbs
37x12.50x17, D, 36.8", 73 lbs (+4 to 35)
Pitbull Rocker
37x12.50x15, 37.00", 88 lbs (+15 to 37x12.50x17 MT/R & KM2)
37x12.50x17, 37.25", 90 lbs
It looks like your 37x12.50x15 PBR tires weigh about 15 lbs each more than both the MT/R or KM2 in a 37x12.50x15. It also looks like the difference between an otherwise identical tire on 15s vs. 17s to be about 3lbs.
I think between the 15 extra pounds per tire, softer rubber compound, the new PBRs being bigger than the worn MTRs, and slightly more give in the sidewall from the 15 inch wheels, I am not surprised that they feel like "dogs" compared to the other tires unless they were of similar weight. I would expect the weight of the tires to be by far the biggest factor. I felt it going from my BFGs to the Coopers.
I don't think the OP would notice a significant difference between a 35x12.50x15 MT/R (67 lbs) and a 35x12.50x17 MT/R (64 lbs) knowing that the 17 inch wheel will weigh a little bit more than a similar 15 would.
