We have quite a few girls in our dojo, and I can understand what you mean. The girls don't strike hard as the boys do and their breakfall isn't quite good... I wish we had a female yudansha to be a role model for them to aspire to, and teach them the ropes from a female perspective (and decrease the testosterone levels in this sausage fest). As we didn't have any females for quite some time, this is a big improvement, but I still hold out hope for a female blackbelt to sudden join us (anybody interested in teaching Yoshinkan Aikido in Vancouver?).
I personally treat them just the same as the boys, striking the same strength, giving good technique and pins. They are here to learn a budo, so you are disrespecting their resolve if you show them any less. Now, you should treat both sexes in a gentle manner, and get their permission before getting rough and hard, just out of respect for their body and health, but of course if a girl were to ask me to strike with all my force, I would do as asked.
Keith Taylor sensei is really good on this aspect too, as he regularly throws the girls around just the same as the boys, to let them feel the true technique as it is (so as long as it's within the acceptable ukemi level).
Now, I just need to go out and recruit some more girls