small BRAIN (and body)

bruh

I wonder why potion of growth is uncommon and potion of diminution is rare.

I can't tell you a lore reason. But a possible game mechanic's reason is you are basically only going to enlarge yourself where you might shrink an enemy or yourself (lighter to get over traps/spells like Telekinesis or smaller to hide or whatnot better). Thus the item with more versatility get's a higher rarity.

Quote from Herodragon33 >>

I can't tell you a lore reason. But a possible game mechanic's reason is you are basically only going to enlarge yourself where you might shrink an enemy or yourself (lighter to get over traps/spells like Telekinesis or smaller to hide or whatnot better). Thus the item with more versatility get's a higher rarity.

Shrinking an enemy with a potion is a rather difficult scenario to make happen. It either requires some clever deception play, designing some kind of trap where you trick an enemy into ingesting the potion, or pinning down a creature and forcing it to swallow the potion somehow. It'd be far easier to simply cast the spell on the target which is already balanced by giving the target a constitution saving throw to resist the effect.

Ultimately, I don't see any reason for this to be a higher rarity than a potion of growth from a mechanics perspective.

However, a spell scroll of enlarge/reduce is also a consumable uncommon magic item and being a spell scroll restricts its use to only those with the spell on their class list.

So balance-wise the potion of growth arguably should also be rare. It lasts 1d4 hours instead of just 1 hour and doesn't require anyone's concentration to maintain and being a potion any creature can drink it regardless of class features.

Shrinking a huge or gargantuan allied creature - let's say, a true polymorphed bronze dragon - would be a major advantage though. It is rather difficult to maneuver a gargantuan creature trough dungeons.