See my link on page one of this thread. It took me a while to decide which size, and I emailed the company about it. Dylan was six months old at the time, and he has filled out more since then, he is a big boy, he weighed 14 kg at the last weight n aged 8 months. He's now coming up for ten months, and it still fits him, there is a lot of room for adjustment on the harness.
The sport version has thicker, softer straps, which I thought would be more comfortable, also reflective patches.
Lesley and Dylan