When it comes to understanding hook sizes, my suggestion is don’t dive too deep. What determines hook size is rather relative and often differs between manufacturers and even hook models.
If there’s any one factor that determines hook size, it’s probably the hook gap but even this isn’t a true measurement. What you really need to know is that hooks are assigned a number by the manufacturer to indicate their size. The larger the number, the smaller the hook and here in the States anyway, these are most often even numbers, with the exception of a size 1. So, the smallest hooks currently available are a size 32 and they go up in size from there. With hooks larger than a size 1, the numbers begin to increase by 1, but with the addition of a backslash and a zero, with the zero pronounced as “aught” as in a 2/0 hook. Some people also say O rather than aught, as in 2/0. These larger hooks will go all the way up to a 20/0 big game hook.