It wasn't really spelt out by JY in the novel but lets examine some facts and timelines:
Young teen (12-14? I cant recall) MRB = Good enough to strike 40 year-long fear in 1st class fighter. Kill them in 1 stroke.
Young XYS (~30 yrs) = Good enough to terrify ~same age MRB. Made MRB piss his pants.
~60 year old XYS = MRB
So while both were at Shaolin, MRB closed that large gap.
Was young XYS that much more talented than MRB since he had some unknown no name teacher and he was so far ahead of MRB with his hyped Murong arts? If he was more talented, why did MRB close that large gap? If they were equally talented, how did MRB close that large gap?
The only argument (I dont't buy it though) could be that MRB (as stated by Sweeper) 'borrowed' a lot more manuals than the couple XYS borrowed. But MRB did not have XWXG so he had to learn em the hard way, it wasn't really stated how many was mastered and even in the novel, it never seemed that learning more of the 72 arts stacks your power.
I think JY missed an opportunity to go with the DGSD theme by letting Sweeper lecture XYS on how his hatred/revenge filled mind and subconcious knowledge of his 'stealing' Shaolin arts actually nerfed his natural development. i.e., He developed fast naturally when he was still a heroic and pro-peace Xia. Once his mind got clouded, it actually hindered him.
We normally go by the rough approximation that 1 Xiao Feng/Yuanshan ~= 5 Xuan Monks
At Yanmen, 1 Xuanci (pls presumably a few of his Xuan buddies) + Wang Jiantong + over 20 'Elite' fighters couldn't even put a scratch on XYS.
1 Yanmen XYS >>>>>> 5 Yanmen Xuan Ci
1 Old XYS only ~= 5 Old Xuan Ci
HE NERFED HIMSELF!!!!!