The "cup size preference" in anime has very little to do with actually preferring some cup sizes over others. It's about more common aesthetic preferences, like submissive vs assertive. Titty monsters are universally tall and have thick thighs, big butts, and imposing hairstyles. Flat girls are short and have a small and round butt (if there're any secondary sexual characteristics).
This generates the titty dynamic. It has only one rule and it states that the flatter mass will yield. In Euphonium the club leader let's the largest pair be the de facto leader, and both of the exceptionally talented girls are also exceptionally "talented". Satsuki is the bully and Ryuko is her target (and Ragyo is the mastermind). Mami is a respectable senpai, but Kyoko is just a nuisance. Nico failed both her original club and her claim to authority while Nozomi is the secret mastermind behind µ's.
Everything contradictory to this rule is a trick act that operates on the fact that things are not as they are presupposed to be. When a flat magical girl beats some buxom invader it shows that besides the raw power and capacity one has, there's an element of justice to fighting and winning (much like how in Saint Seiya Shiryu beats Deathmask by simply being more just). The same goes for overpowered lolis like Nue, Tatsumaki, and Flandre - they defy common sense and physical limitations, which is a great device to underline how ridiculously powerful they are. In Madoka Magica surprises and "deconstruction" are central, so the power levels rise from largest to smallest.
Bottom line / tl;dr: Anime should respect its own conventions, just like every other medium.