But then it is true that women are generally frailer, smaller and easier to unbalance
Although women may be generally smaller and less muscular, I don't agree that they are easier to unbalance!
In my experience female yudansha (1st - 6th dan) that I have trained with are generally harder to unbalance than the men of the corresponding rank
I think this is because we are physically able to get the whole dropping of centre and sinking of weight thing (due to our build) better than the guys, who tend to use a bit too much upper body even at yudansha level..
I have also noticed this amongst some upper kyu ranked female students, when they get the balance right and stop trying to copy the upper body led techniques and ukemi of the men
To the OP - I don't think there is anything your Sensei can do to increase the number of female students - people (regardless of gender) will quickly decide whether or not Aikido is for them, and although you can offer it to them, you cannot decide for them.