I doubt it was because it was too big. My little pied girl has eaten rat pups and last week a weaned rat that was twice her weight and she was fine. She looked like she swallowed a football both times and almost went into a food coma before making it back into her warm hide but she was fine. If it was F/T, I would make sure they are fully thawed and warmed before feeding and make sure the snakes temps and everything are good. It could be from stress depending how easily your snake is stressed out. my little girl lets me pick her up while she is eating as she seems to love to slide her rats off the paper plate so I have to pick her up and pick off the aspen on the rat while shes eating it and then put her and the rat back down on the plate. She doesn't even care and keeps swallowing during the whole ordeal My red tail boa is the same way but I cant pick her fully up as she is 6ft, I just pick her front half up and put it and the rat in her feeding dish to keep aspen off it. Are you offering your snake good hides and making it feel secure? When I got the little BP girl, she was head shy and very timid like the pet shop was mean to her but now after almost 3 months, she isn't head shy at all and loves to sit in a loose coil on my belly/chest and just let her head rest of my arm for hours infront of the tv. Just make sure you give them time to adjust and be nice and slow with them and let them get to know you instead of you forcing your will on them.

As for how much to feed, general rule of thumb is 10-15% of their weight every 3-5 days for hatchlings or some people say up to 500g. Then after that go to a 1 week feeding of something like a small rat. At that time you don't go by weight anymore. I got Dot when she was 102g and I fed her 10-20% every 3-4 days and now she is 420g as of today. So she gained about 318g in almost 3 months. She also used to fly out of her hides like a cobra at the smell of a rat but now with all this time of good feeding, she no longer acts like shes starving to death. Today I fed her a large rat pup almost weanling and she poked her head out and grabbed it and gobbled it down but she wasn't acting all starving.