Hey Tan

I do tend to upsize with nappies, just because I feel like if I get the 'smaller' size they seem to grow out of it so quickly.
The thing to remember though is that you can only upsize to a certain point, and you'll probably find that some nappies you can upsize and with others you can't so much. If you do go for the larger size you just need to make sure that you can still do it up firmly up around their legs and waist (as that's where you might get leaks).
I find that with a lot of styles we have tried, although they state the weight range there is a bit of flexibilty outside of that.