>>1873022I actually think it's similar to what both
>>1917067 and
>>1873448 said in that both traits lend to the submissive/femininity of the girl. As a fan of both the large breasts and glasses fetishes, there is a considerable amount of overlap and it either takes the form of an older woman who is drawn to look more developed and the glasses make her look smart or mature..., or a young and helpless/feminine girl and the glasses make her appear more submissive or prone while still being obviously sexual due to the large breasts. A young flat chested girl with glasses just looks like a girl with glasses and no subtext so it hits a smaller market of appeal and the sexualization of the girl has to be more subvert or complex than just "big boobs" for it to incite a similar pornographic interest.
Big boobs are so easy and prevalent as a sexual indicator that it seems only natural to associate it with a subject reflecting either maturity or helplessness as it too reflects those traits most of the time. (I.e. the busty girl is always shy or the teacher is always curvy)
That's just my analysis ¯\_(ツ)_/¯
