In the overall picture, nearly every asset in BG2 is geared towards one thing: combat. As sad (and still rectifiable) as it is, there are scant few instances in dialogs or plots where a character’s attributes, skills, or even race or class, come into play. This is certainly the case with magic. This makes magic a necessarily truncated experience, as the things that can be altered and the ways those things can be altered are both limited: thac0, damage, AC, saves, and various states like panic, hold, and charm. All spells outside of the metamagic category are some combination of these effects as either boons to allies or banes to enemies. Some effects like petrification have a minimum but also static level of effectiveness (one cannot be partially petrified). Others like panic have a de facto maximum due to widespread immunities or easy to obtain counters. Both types can be given power mobility by adjusting the saving throws of their effects, and limiting or expanding their range or area of effect. Other effects, notably damage, are easily scalable and can be crafted to fit any situation. In essence, there are only a handful of spells, but by managing the likelihood and extent of these handful of effects, we can have seemingly disparate spells of differing ranks. (I use the term “rank” instead of “level” when talking about spell levels… er, ranks.)
One way to go would be to simply enumerate all these spells at rank One and scale them thereafter with no need for Two through Nine other than as deliberately expanded options. Take Charm. At rank 1, for example, it affects one creature of a specific creature type (humanoid) for a short amount of time with a significant save bonus. That same rank 1 spell could scale to gradually reduce the save bonus into a penalty, the duration could be lengthened, the target types affected could eventually become universal, and the number of targets increased. That’s one scaled spell that is doing the job of multiple BG2 spells without any real loss of verisimilitude or variety. BG2/AD&D just so happened to parse out these iterations in a more arbitrary fashion. Many damage spells do this exact thing but it seems more natural when the scaled effect is ‘damage’ as opposed to a status effect.