That bigger comb looks about right for a female at 18 weeks, getting ready to lay eggs soon.
Chickens can have big differences in size, even within the same breed, so I mostly ignore size when trying to figure out gender.
Does it crow? I would expect a male to be crowing by this age.
I cannot see the saddle area very clearly in the pictures. Its on the back of the chicken, just in front of the tail, and roosters grow distinctive feathers there (long and slender, with pointy ends that eventually hang down on both sides.) I cannot tell whether it has saddle feathers starting to grow or not. I would expect saddle feathers to be pretty well developed by 18 weeks, unless you have a male that is unusually slow to mature.
Considering all those things, I think it's probably a female, but I can't be certain without a better picture of the saddle area on its back.
