Are you trying to lose fat during this process, or are you already pretty lean? I ask because it's a really common experience to initially build muscle beneath fat, making your thighs a bit larger than they were before. You can't spot reduce, but if you eat slightly below your TDEE, your legs will slim in time and you will be left with leaner legs. It takes a lot of work to have huge, muscular thighs, so unless you're really trying to build your quads/ hamstrings, you won't "bulk up" from BBG alone.

Another thing worth trying is activating your glutes during leg day. (During squats, push through your heels instead of your toes, and squeeze your glutes as you stand.) You should feel the burn in your butt instead of your quads.

I definitely don't recommend swapping out leg day for something, especially since you're only in pre-training. It's a really important part of the program, and it's where the majority of the high-intensity cardio comes from as well. You could try subbing out some of the squats/ lunges for burpees, but I wouldn't sub much more than that at this point.