I believe every BP grows at a different rate. I've seen perfectly healthy yearlings at 1000+ grams and some that barely even break 600 grams. I think what most people on here will tell you is that as long as your BP is eating fine and not losing any noticeable weight, you're for the most part fine. I'm still new to the hobby so this is merely my observations while I was researching answers myself.

What I've researched in terms of feed size was, while they're younger, I think girth and size matters more than actual weight. This is again, pure observation of the information I had collected but weight is more of an important factor once they reach maturity. I think as long as you size up your rats/mice in relation to your BP's widest girth point, you should be fine.

Please feel free to chime in and correct me if my information is wrong though.