The bug

If an item model's scale is left unspecified or set to the default value of [ 1, 1, 1 ] manually, it will appear the size of a full block in most cases. However, this does not occur for items placed in item frames or in the head equipment slot.

The following table lists results:

Display type	Use case	Result for default scale
gui	Inventory items	Full slot
head	Items equipped on the head	Smaller than a full block
ground	Dropped items	Full block
fixed	Item frames	Smaller than a full block
thirdperson_lefthand	Items held in an entity's left hand	Full block
thirdperson_righthand	Items held in an entity's right hand	Full block
firstperson_lefthand	Items held in the player's left hand in the UI	Probably a full block
firstperson_righthand	Items held in the player's right hand in the UI	Probably a full block

How to reproduce

A resource pack is attached to this ticket that creates custom item models for cobblestone and mossy cobblestone, without referencing any parent models. Cobblestone's model does not specify anything for item display settings, whereas mossy cobblestone's model explictly specifies the default values.

The /item command can be used to replace the head item of a target entity such as a zombie or armor stand with cobblestone or mossy cobblestone.

Expected results

With this resource pack applied, cobblestone/mossy cobblestone equipped on the heads of entities and placed within item frames would appear the size of a full block.

Actual results

In both cases, they appear smaller than a full block.