My two cents and I have not smoked a 20+ year old Cuban cigar but I drool with anticipation
I believe most blenders target an optimal size for a blend and the size varies. Illusione blends are initially designed around the corona size so it's "optimized". Then the other sizes are tweaked usually by adding more filler which I believe why large ring gauge cigars taste washed out and bland. In the last few years 60 RG cigars became vogue so new blends are optimized around the larger ring gauge and the smaller RG in the newer cigars may appear lacking.
The best advice I could offer is try various sizes in the brand and see which size appeals to you more.
My tastes enjoy the corona sizes from Illusione, Toros from Don Pepin blends and Viaje. I would love to hear others as they weigh in? I will never claim I have a good palate so take what I say as just my two cents