though buckets logically should ... they're clearly shaped so they'd nest.
This is because buckets are wearable items. Due to that, they can't properly stack. If they made seperate items for wearing like with the cooking pan helmet (although oddly enough the cooking pan not wearable doesn't stack either), they could properly stack and elevate a lot of pain.
As long as all buckets remain helmet options, I don't think it's possible mechanical-wise.
It isn't even max stacks, as sometimes I get three stacks, one of maybe 2, another of 3 and a third of 10. (That example is currently pulled from the blue mushrooms in my bags.)
Yeah... I also get the I've got 2 stacks of the exact same item for some reason.
I understand now why for the skulls... some items have more expensive varities (for which I currently don't see the reason besides more clutter... unless they do make a difference in crafting I miss?). Though it could be some stack maximums are extremely weird. For example potions are 99, but feathers for some weird reason are 50. It could be others have even lass of a stack-maximum... for no reason what-so-ever.
Or it may have another reason... still gathering more ingredients for a proper theory.