Will jogging/running on the treadmill make my thighs bigger or slimmer? thanks!

Thunder thighs??: Probably neither if your female hormones are sufficient. For real build up you need more direct stress like squats with a heavy barbell or similarly stressful machine work out. If you are overweight slimming you legs is better accomplished with diet AND exercise. If you wish to avoid "skinny legs" weight lifting and leg resistant exercises would be ideal.