My third started sleeping through the night at like 14 months. It has nothing to do with weight or age, it is 100% dependent on the kid. My routines were exactly the same with my second and she slept through the night great starting at 3 months.
Sometimes kids are just jerks.
Oh and latest thing in my house... my 6 year old has developed the hugest martyr complex lately. The entire world is against her these days. She actually said "you're ruining my life" today and my husband and I just cracked up laughing. I didn't think I'd hear that for another 7 years at least. She was all on about how we are "stifling her sense of adventure" (her words) because I told her not to run down the street by herself when we were getting in the car. Like... wtf? I wasn't able to keep a straight face during this tantrum which drove her even crazier. I guess I'm a bad mommy.. but my god it was ridiculous.