The number of people sitting exams is not a good estimate of intake difficulty.
More objectively one should look at the last ranking position that got in the school as percentile of total number of students that took the exam.
WGCS is part of the SET exam, and compared to Wilson and SGS, every year WGCS typically go down to very deep of the waiting list for the last applicant to get in the school. Generally SET pass scored is over 30% of the total applicants (around 2000+ boys), the guess is WCGS last ranking position via normal academic selection route got in is around 30% percentile.
Not idea about RGS last rank percentile vs total applicant, but I guess maybe similar to top 30%, could be lower, but with a slightly different exam cohort due to location difference.
Some top superselctive grammar might get last ranking position as high as 10%, such as QE boys or HBS. The "top" private selective school in UK probably won't get last ranking position as high as 10%, maybe highest is around top 15-20%.
With due respect WCGS is a good grammar school, but not a superselective top grammar in terms it intake difficulty.
If one really want to compare the WCGS vs RGS in terms of academic aspect in general , as a suggested earlier, best way is to look at the public exam results these school achieved. Clearly on average RGS is ahead of WCGS consistently in the past years.
One might argue public exam performance might not be entirely due to the academic quality provides by the school, another factor is how selective these schools are in the first place. Then your argument of WCGS take "more academic inclined" kids than RGS is contradict itself as RGS shows better public exam results in the end.