If you want good separation between your quadriceps and hamstrings, first make sure you are concentrating and that you’re really feeling weight on each rep. That is how you will create more detail. If you lift with explosive contractions, you won’t really fell the muscle, just the weight. You will get bigger but not necessarily better separation. For legs, use high repetitions on drop sets to fully engage and fatigue the muscles. Aim to do more than 15 reps per set so that you feel the burn. Plus, high rep training for legs helps to burn through more glycogen (stored sugars in the muscles) and total calories to help you get leaner overall.
I started lifting to lose weight. I felt I was making progress, but I just weighed myself for the first time in a month and found I have actually gained weight. What am I doing wrong?
The only mistake you are making is letting the scale dictate how well you are doing. Muscle weighs more than fat, so if you are training hard you should expect to gain weight in the form of muscle. Even though you are gaining weight, you are most likely looking better overall. Focus on what the mirror shows. If you are looking better and you are leaner, that is a positive result. You are swapping fat for muscle, proof that your program is working.