I don't think the pebble's reflex really even matters here.
The PC is trading the primary target damage (as we assume they don't care how much they hurt said pebble) in order to splash damage targets around them. It seems to be a fairly legitimate and even 'real life' tactic. It's the same thing you do with any other AOE spell - in fact, this only seems to be hurting the PCs, because while the initial attack is 'easier', they're losing the 'primary target' of the spell.
In reference to size categories, it may be worth bringing up semantics on how big the 'force orb' actually is that you are throwing. Maybe the reason why pebble size vs bottle size vs whatever doesn't come into play is because it's irrelevant whether you can target something smaller.
IE - if I throw a basketball at my daughter's toy tree house (roughly the size of the basketball) I hit it. Why would I need to target the dolls inside - if I am able to succeed on the larger target, the smaller one is hit by default because the projectile is too large. Anyway, point is, I'm not familiar with the sizing on Force Orb, but if the 'projectile' orb is too large, it won't matter.