I wish I could post a picture of my sister's butt. I seriously want it! She attributes its awesome shape from squats/lunges and dancing... So I wouldn't necessarily say lunges and squats make your backside "bigger or smaller," but it WILL look more toned and in shape. Even my ass looks noticeably different than before I started working out regularly.
To maximize squats/lunges, use free weights and do bicep curls or shoulder raises as you're doing the movement. It will burn more calories than just a squat/lunge alone, making it more effective.
Increased muscle mass comes not from the exercises itself but from how frequently it's worked, the level of resistance, and the amount of reps. Bodybuilders will only work the same muscle group once a week. People just looking for toning will do the same exercises as bodybuilders but work it 3-4 times a week. Bodybuilders will do 5 sets of 5 reps with the bar loaded up with the maximum amount of weigh tthey can possibly handle. People looking for toning will do 15 reps per set with only a little bit of added resistance. Same exercises, different results, and it comes from the rest you give it, amount of resistance, and reps. That being said, if you perform any sort of resistance training on the same muscle group with less than 48 hours rest, you are risking severe muscle injuries that you may not ever be able to fully recover from.
They're low intensity so they'll build lean muscle which won't get too bulky, just toned. If your butt is small it might get bigger and if it gets big it might get smaller. Otherwise, It'll probably just sculpt the shape a bit.