Maybe less EXP for bigger parties and more for smaller ones
That's exactly how it works in D&D and in the old BG. I'm sure Larian know it.
More specifically, since in Larian games basically only the main character levels up and others simply keep up with him, what you are supposed to do is to adjust how much exp he gets according to how many partners he carries around. Which is the norm. People talking about it as an obscure method that would require the longest, most elaborate inspection when it has been tried and tested over YEARS of practical use are ridiculous.
Vaguely legitimate worries, if any, would rather be that: - you can eventually exploit this system levelling up faster and THEN grouping more companions anyway. - at some point all characters are going to hit a level cap anyway.
To both the most appropriate response is "SO FUCKING WHAT?" It's irrelevant. Just more fake concerns, in practical terms. These games are never "finely tuned" to make possible barely edging them. There's always a massive headroom making them completable at any skill level with a far from optimal setup.