bummis swim diaper sizing

just.a.lurker

Anyone have experience with how true to size these run?  Trying to decide between the medium and large for DS.  He's tiny, just under 16lbs right now.  The medium says 15-22lbs, and large is 22-30.  I'm leaning more towards the large since it will be later this spring and summer that he will use it, but wasn't sure if they run huge.  Any input?