Not sure why this is happening, but I would assume its because he is walking and getting larger? Recently I stopped using snappis because I felt that my LO was growing out of them and they weren't holding tight enough (I was primarily using them on a bamboo fitted for nighttime. It wasn't holding well enough to get our snug wool soaker on over it). I wasn't aware that there are 2 sizes for snappis. I got a pair of boingos which are much bigger than I pictured, but didn't grasp the bamboo well, so for nights now I am using pins. They work so much better.

I haven't tried boingos with a prefolds so can't comment on the holding power, but I think they would definitely work better if your problems are related to the snappi being too small or it coming loose when your guy is walking. Right now I don't fasten my prefolds at all. I let the cover hold it in place. My son is 5 mo though and really not mobile.

Just put your fingers inside the diaper so if anyone gets poked it will be you. I used nothing but pins for years back when it was basically the only option and I never poked my daughter.