As the parent of a girl chess player, I would say that yes- numbers play a huge part. When she goes to competitions, there may be three groups in the boys' age sections, whereas the girls are often struggling to get even a whole group in each age, and usually put two (or even 4) age categories together.
She's in upper KS2, an age where there are vast differences socially between boys and girls, and in mixed matches, she is always intimidated and outright bullied by boy opponents. You see it standing in the halls- the girls will greet one another, shake hands, sit down quietly, chat before the game starts about where they go to school, or whether they were in that championship last year, etc, etc. The boys often very grudgingly shake hands, their body language is very negative and they take an aggressive stance. In team matches, teams entirely of boys behave as a pack, sometimes all kicking the chairs under the table throughout a match, cutting off any approaches verbally etc.
She has had far too many boys "flip the board" when they're beaten by a softly-spoken, polite little girl, and they storm off, complaining of cheating, outraged, etc. In short, boys do not want girls in their sphere, and they will do whatever it takes to keep them out.
I guess as girls get older, the desire to have a nice time playing their hobby overtakes the desire of the win, and more and more drop out. Often at competitions, there will be only 1 girl entrant in each age category (which DD likes, as they automatically get the shield/money, etc.).